Filter material construction and method

ABSTRACT

A preferred filter media is provided. The media includes a fine fiber web secured to the surface of a coarse fiber support. A preferred filter media, comprising multiple layers of fine fiber media separated by coarse fiber support, is provided. Advantageous filter constructions result and are provided. Also according to the disclosure, methods for using such arrangements to filter are provided.

1. Field of the Invention

The present invention relates to filters, filter constructions,materials for use in filter constructions and methods of filtering.Applications of the invention particularly concern filtering ofparticles from fluid streams, for example from air streams. Thetechniques described herein particularly concern the utilization ofarrangements having one or more layers of fine fibers in the filtermedia, to advantage.

2. Background of the Invention

Fluid streams such as air and gas streams often carry particulatematerial therein. In many instances, it is desirable to remove some orall of the particulate material from the fluid stream. For example, airintake streams to the cabins of motorized vehicles, to engines formotorized vehicles, or to power generation equipment; gas streamsdirected to gas turbines; and, air streams to various combustionfurnaces, often include particulate material therein. In the case ofcabin air filters it is desirable to remove the particulate matter forcomfort of the passengers and/or for aesthetics. With respect to air andgas intake streams to engines, gas turbines and combustion furnaces, itis desirable to remove the particulate material because it can causesubstantial damage to the internal workings to the various mechanismsinvolved.

In other instances, production gases or off gases from industrialprocesses or engines may contain particulate material therein. Beforesuch gases can be, or should be, discharged through various downstreamequipment and/or to the atmosphere, it may be desirable to obtain asubstantial removal of particulate material from those streams.

A variety of fluid filter arrangements have been developed forparticulate removal. For reasons that will be apparent from thefollowing descriptions, improvements have been desired for arrangementsdeveloped to serve this purpose.

A general understanding of some of the basic principles and problems ofair filter design can be understood by consideration of the followingtypes of media: surface loading media; and, depth media. Each of thesetypes of media has been well studied, and each has been widely utilized.Certain principles relating to them are described, for example, in U.S.Pat. Nos. 5,082,476; 5,238,474; and 5,364,456. The complete disclosuresof these three patents are incorporated herein by reference.

In general, for any given application, filter design has typicallyconcerned a trade off of features designed for high filter efficiencyand features designed to achieve high capacity (i.e. long filterlifetime). The "lifetime" of a filter is typically defined according toa selected limiting pressure drop across the filter. That is, for anygiven application, the filter will typically be considered to havereached its lifetime of reasonable use, when the pressure buildup acrossthe filter has reached some defined level for that application ordesign. Since this buildup of pressure is a result of load, for systemsof equal efficiency a longer life is typically directly associated withhigher capacity.

Efficiency is the propensity of the media to trap, rather than pass,particulates. It should be apparent that typically the more efficient afilter media is at removing particulates from a gas flow stream, ingeneral the more rapidly the filter media will approach the "lifetime"pressure differential (assuming other variables to be held constant).

Paper filter elements are widely used forms of surface loading media. Ingeneral, paper elements comprise dense mats of cellulose fibers orientedacross a gas stream carrying particulate material. The paper isgenerally constructed to be permeable to the gas flow, and to also havea sufficiently fine pore size and appropriate porosity to inhibit thepassage of particles greater than a selected size therethrough. As thegases (fluids) pass through the filter paper, the upstream side of thefilter paper operates through diffusion and interception to capture andretain selected sized particles from the gas (fluid) stream. Theparticles are collected as a dust cake on the upstream side of thefilter paper. In time, the dust cake also begins to operate as a filter,increasing efficiency. This is sometimes referred to as "seasoning,"i.e., development of an efficiency greater than initial efficiency.

A simple filter design such as that described above is subject to atleast two types of problems. First, a relatively simple flaw, i.e.rupture of the paper, results in failure of the system. Secondly, whenparticulate material rapidly builds up on the upstream side of thefilter, as a thin dust cake or layer, it eventually substantially blindsoff or occludes portions of the filter to the passage of fluidtherethrough. Thus, while such filters are relatively efficient, theyare not generally associated with long lifetimes of use, especially ifutilized in an arrangement involving the passage of large amounts offluid therethrough, with substantial amounts of particulate material ator above a "selected size" therein; "selected size" in this contextmeaning the size at or above which a particle is effectively stopped by,or collected within, the filter.

Various methods have been applied to increase the "lifetime" ofsurface-loaded filter systems, such as paper filters. One method is toprovide the media in a pleated construction, so that the surface area ofmedia encountered by the gas flow stream is increased relative to aflat, non-pleated construction. While this increases filter lifetime, itis still substantially limited. For this reason, surface-loaded mediahas primarily found use in applications wherein relatively lowvelocities through the filter media are involved, generally not higherthan about 20-30 feet per minute and typically on the order of about 10feet per minute or less. The term "velocity" in this context is theaverage velocity through the media (i.e., flow volume÷media area).

In general, as air flow velocity is increased through a pleated papermedia, filter life is decreased by a factor proportional to the squareof the velocity. Thus, when a pleated paper, surface loaded, filtersystem is used as a particulate filter for a system that requiressubstantial flows of air, a relatively large surface area for the filtermedia is needed. For example, a typical cylindrical pleated paper filterelement of an over-the-highway diesel truck will be about 9-15 inches indiameter and about 12-24 inches long, with pleats about 1-2 inches deep.Thus, the filtering surface area of media (one side) is typically 37 to275 square feet.

In many applications, especially those involving relatively high flowrates, an alternative type of filter media, sometimes generally referredto as "depth" media, is used. A typical depth media comprises arelatively thick tangle of fibrous material. Depth media is generallydefined in terms of its porosity, density or percent solids content. Forexample, a 2-3% solidity media would be a depth media mat of fibersarranged such that approximately 2-3% of the overall volume comprisesfibrous materials (solids), the remainder being air or gas space.

Another useful parameter for defining depth media is fiber diameter. Ifpercent solidity is held constant, but fiber diameter (size) is reduced,pore size is reduced; i.e. the filter becomes more efficient and willmore effectively trap smaller particles.

A typical conventional depth media filter is a deep, relatively constant(or uniform) density, media, i.e. a system in which the solidity of thedepth media remains substantially constant throughout its thickness. By"substantially constant" in this context, it is meant that onlyrelatively minor fluctuations in density, if any, are found throughoutthe depth of the media. Such fluctuations, for example, may result froma slight compression of an outer engaged surface, by a container inwhich the filter media is positioned.

Gradient density depth media arrangements have been developed. Some sucharrangements are described, for example, in U.S. Pat. Nos. 4,082,476;5,238,474; and 5,364,456. In general, a depth media arrangement can bedesigned to provide "loading" of particulate materials substantiallythroughout its volume or depth. Thus, such arrangements can be designedto load with a higher amount of particulate material, relative tosurface-loaded systems, when full filter lifetime is reached. However,in general the tradeoff for such arrangements has been efficiency,since, for substantial loading, a relatively low solids media isdesired. Gradient density systems such as those in the patents referredto above, have been designed to provide for substantial efficiency andlonger life. In some instances, surface-loading media is utilized as a"polish" filter in such arrangements.

SUMMARY OF THE INVENTION

According to certain aspects of the present invention, a filter mediaconstruction is provided. The filter media construction can be used as afilter media in preferred filter arrangements. It may, in someinstances, be utilized as one layer of media in a multi-layerarrangement, for example. In some arrangements, layers of filter mediaaccording to the present invention can be stacked, to create a preferredconstruction.

A preferred filter media construction according to the present inventionincludes a first layer of permeable coarse fibrous media having a firstsurface. A first layer of fine fiber media is secured to the firstsurface of the first layer of permeable coarse fibrous media. Preferablythe first layer of permeable coarse fibrous material comprises fibershaving an average diameter of at least 10 microns, typically andpreferably about 12 (or 14) to 30 microns. Also preferably the firstlayer of permeable coarse fibrous material comprises a media having abasis weight of no greater than about 50 grams/meter², preferably about0.50 to 25 g/m², and most preferably at least 8 g/m². Preferably thefirst layer of permeable coarse fibrous media is at least 0.0005 inch(12 microns) thick, and typically and preferably is about 0.001 to 0.010inch (25-254 microns) thick.

In preferred arrangements, the first layer of permeable coarse fibrousmaterial comprises a material which, if evaluated separately from aremainder of the construction by the Frazier permeability test, wouldexhibit a permeability of at least 150 meters/min, and typically andpreferably about 200-450 meters/min. Also preferably, it is a materialwhich, if evaluated on its own, has an efficiency of no greater than 10%and preferably no greater than 5%. Typically, it will be a materialhaving an efficiency of about 1% to 4%. Herein when reference is made toefficiency, unless otherwise specified, reference is meant to efficiencywhen measured according to ASTM #1215-89, with 0.78μ monodispersepolystyrene spherical particles, at 20 fpm (6.1 meters/min) as describedherein.

Herein, when a layer of material utilized in arrangements according tothe present invention is characterized with respect to properties it"has" or would exhibit "on its own" or when tested "separately from theremainder of the construction", it is meant that the layer of materialis being characterized with respect to the source from which it isderived. That is, for example, if reference is made to the "coarse"layer of material, in a composite, the description when characterized asreferenced above, is with respect to the material and its properties asit would have existed before being incorporated into the construction.Reference in this context is not necessarily being made to the specificnumerical characteristics of, or performance of, the layer as itoperates in the composite structure.

Preferably the layer of fine fiber material secured to the first surfaceof the layer of permeable coarse fibrous media, is a layer of fine fibermedia wherein the fibers have average fiber diameters of no greater thanabout 10 microns, generally and preferably no greater than about 8microns, and typically and preferably have fiber diameters smaller than5 microns and within the range of about 0.1 to 3.0 microns. Also,preferably the first layer of fine fiber material secured to the firstsurface of the first layer of permeable coarse fibrous material has anoverall thickness that is no greater than about 30 microns, morepreferably no more than 20 microns, most preferably no greater thanabout 10 microns, and typically and preferably that is within athickness of about 1-8 times (and more preferably no more than 5 times)the fine fiber average diameter of the layer.

Preferably, when the application is for air filter applications such asengine induction systems, gas turbines, cabin air filtration, and HVAC(heat, ventilation and air conditioning) systems, the preferred upperbasis weights for the fine fiber layers are as follows: for a layer ofglass fiber material average size 5.1 micron, about 35.8 g/m² ; forglass materials average fiber size 0.4 micron, about 0.76 g/m² ; and,for glass fibers average size 0.15 micron, about 0.14 g/m² ; forpolymeric fine fibers average size 5.1 micron, about 17.9 g/m² ; forpolymeric fibers average size 0.4 micron, about 0.3 g/m² ; and, forpolymeric fine fibers 0.15 micron average size, about 0.07 g/m². Ingeneral, preferably the most upstream layers of fine fibers has a basisweight of no greater than about 1 g/m², for such applications.

When the material is utilized for high efficiency applications, such asselected indoor air applications and liquid applications (such as lubeoil, hydraulic fluid, fuel filter systems or mist collectors), ingeneral the preferred upper limits of the basis weights for the finefiber layers will be as follows: for glass fibers average size 2.0micron, about 15.9 g/m² ; for glass fibers average size 0.4 micron,about 1.55 g/m² ; and, for glass fibers average size 0.15 micron, 0.14g/m² ; for polymeric fine fibers average size 2.0 micron, about 8.0 g/m²; for polymeric fibers average size 0.4 microns, about 0.78 g/m² ; and,for polymeric fibers average size 0.15 microns, about 0.19 g/m². Ingeneral, preferably the most upstream layer of fine fibers has a basisweight of no greater than about 1 g/m², for such applications.

The upper limits given for the air filtration applications, such as airinduction systems etc., were based upon fine fiber layer thicknesses ofabout 5 fiber diameters, and an LEFS efficiency of 50% for the layer.For the high efficiency applications, the assumption was based upon fivefine fiber thicknesses and an LEFS efficiency of about 90% per layer.

In general, the preferred basis weight for any given situation woulddepend upon such variables as: the application involved (for examplecoarse or fine particles, or both, to be trapped in operation, highefficiency or lower efficiency needs); the desired life; the fibermaterial selected; and, the fiber size used. In general, when relativelyhigh single-layer efficiency is desired (for example on the order of 90%LEFS), generally the glass fibers will work well, and the system willinvolve higher basis weights (for example about 20 g/m²), at higherfiber diameters (for example 2-3 microns).

On the other hand, when relatively low single-layer efficiencies aredesired, but relatively high lifetime until loaded (resulting from theuse of a number of layers) relatively low efficiencies for any givenlayer will be used (for example on the order of 10% LEFS). This willinvolve relatively low basis weights and fairly small diameter fiber.Polymer fibers may be usable for this (although glass ones could also),and thus basis weights on the order of 0.005 g/m², with a fiber size ofabout 0.2 microns will be usable. Herein, when the basis weights aregiven, for glass fibers the assumption is a density of 2.6 g/cc, and forpolymer fibers the assumption is a density of 1.3 g/cc.

In general, then, if what is desired by the engineer is to providelonger life, generally more layers, each layer having relatively lowefficiency, will be used. If the engineer desires a very high efficiencyfilter, and long life is not necessarily desired, in general fewerlayers with higher LEFS efficiency per layer will be used.

Herein the term "first" or "second" in reference to a construction, forexample surfaces of media, is not meant to refer to any particularlocation in the media. For example, the term "first surface" on its ownis not intended to be indicative of whether the surface referred to isupstream or downstream of other surfaces, or positioned above or belowother surfaces. Rather, the term is utilized to provide for clarity inreference and antecedent basis. The term "1-8 fine fiber averagediameters" is meant to reference a depth or thickness of about 1 timesto 8 times the average diameters of the fine fibers in the fine fiberlayer referenced.

In typical preferred systems, the fine fibers of the first layer of finefiber media comprise fibers with diameters of no greater than about1/6th, preferably no greater than about 1/10th and in some instancespreferably no greater than about 1/20th of the diameters of the fibersin the first layer of permeable coarse fibrous media.

For certain applications, preferably the first layer (most upstream inoperation) of fine fiber material is constructed and arranged to providethe resulting composite (i.e. the combination of the first layer ofpermeable coarse media and the first layer of fine fiber media) with anoverall LEFS efficiency of at least 8%, preferably at least 10%,typically within the range of 20 to 60%, and most preferably at least30% and no greater than about 70%. Such composites can then be stackedto create very efficient, for example greater than 97%, filters. Theymay also be used for less efficiency but very long life filters, forexample 50-97% efficient. Also, preferably, the first (most upstream inoperation) layer of fine fiber media is constructed and arranged suchthat the resulting composite (i.e. the combination of the first layer ofpermeable fibrous media with the first layer of fine fiber mediathereon) has an overall permeability of at least 20 meters/min, andtypically and preferably about 30 to 350 meters/min. Herein the term"most upstream" or "outermost" in connection with a fine fiber layerrefers to the layer of fine fiber material (average fiber diameter lessthan 8 microns) in the position to be most upstream, relative to otherfine fiber layers, in use. There may be more upstream layers of media(not fine fiber) than the most upstream fine fiber layer.

The first layer of permeable coarse fibrous material may be fibersselected from a variety of materials, including for example polymericfibers such as polypropylene, polyethylene, polyester, polyamide, orvinyl chloride fibers, and glass fibers.

According to certain aspects of the present invention, a filterconstruction is provided which includes more than one layer, andpreferably at least 3 layers, of fine fiber material. Typically thearrangements will include three or more such layers. It is not arequirement that the fine fiber layers in such a multi-layered system beidentical to one another. However, preferably each fine fiber layer is alayer within the general description provided above for the first layerof fine fiber media in the media construction as described. Preferablyin such arrangements each layer of fine fiber material is separated fromits next adjacent layer of fine fiber material, by a layer of permeablecoarse fibrous material.

The layers of permeable coarse fibrous material need not be identical,but preferably each is within the general description above with respectto the filter media construction, for the first layer of permeablecoarse fibrous media. In certain preferred arrangements, the overallcomposite media construction also has a layer of permeable coarsefibrous media, as described, on both the most upstream and mostdownstream surfaces.

The filter construction may comprise a pleated arrangement of thecomposite, if desired. For example, such an arrangement can have pleatsthat are 0.25 to 12 inches (0.6-30.5 cm) deep, with a pleat density ofat least 1-15 pleats/inch (1-15 pleats/2.5 cm). When it is said that thepleat density is at least 1-15/inch, and the arrangement is configuredin a cylindrical pattern, with the pleats extending longitudinally,reference is made to pleat spacing around the inner diameter or surface.

Certain preferred arrangements according to the present inventioninclude media as generally defined, in an overall filter construction.Some preferred arrangements for such use comprise the media arranged ina cylindrical, pleated configuration with the pleats extending generallylongitudinally, i.e. in the same direction as a longitudinal axis of thecylindrical pattern. For such arrangements, the media may be imbedded inend caps, as with conventional filters. Such arrangements may includeupstream liners and downstream liners if desired, for typicalconventional purposes. The constructions may be utilized in associationwith inner wraps or outer wraps of depth media, for example inaccordance with the arrangements described in U.S. patent applicationNo. 08/426,220, incorporated herein by reference.

It is foreseen that in some applications, media according to the presentinvention may be used in conjunction with other types of media, forexample conventional media, to improve overall filtering performance orlifetime. For example, media according to the present invention may belaminated to conventional media, be utilized in stack arrangements; orbe incorporated (an integral feature) into media structures includingone or more regions of conventional media. It may be used upstream ofsuch media, for good load; and/or, it may be used downstream fromconventional media, as a high efficiency polishing filter. The manyvariations possible will be apparent, from the more detaileddescriptions below.

Certain arrangements according to the present invention may also beutilized in liquid filter systems, i.e. wherein the particulate materialto be filtered is carried in a liquid. Also, certain arrangementsaccording to the present invention may be used in mist collectors, forexample arrangements for filtering fine mists from air.

According to the present invention, methods are provided for filtering.The methods generally involve utilization of media as described toadvantage, for filtering. As will be seen from the descriptions andexamples below, media according to the present invention can bespecifically configured and constructed to provide relatively long lifein relatively efficient systems, to advantage.

As will be apparent from the above discussions, and the detaileddescription below, certain specifically preferred arrangements,especially preferred for air filter constructions, are provided. A formof these is characterized as filter media constructions. The preferredfilter media constructions comprise a plurality of layers of fine fibermedia, i.e. at least two layers, each of the layers of fine fiber mediacomprising fibers having diameters of no greater than about 8 microns.The plurality of layers of fine fiber media include an outermost layer.Again, by "outermost" in this context, it is meant that there is a layerof fine fibers in the media which, when the media is organized ororiented for use as a filter media, would be positioned more upstreamthan any other layer of fine fiber material. This does not mean that thefirst "outermost" layer of fine fiber material is the outermost layer ofmedia in the construction. Rather, it is the "outermost" or end layeramong the plurality of fine fiber layers. When this filter mediaconstruction is in use, then, this fine fiber layer will be the upstreamfine fiber layer of media in the construction. Preferably this outermostlayer of fiber fibers includes fibers having an average diameter of nogreater than about 5 microns, and a thickness of no greater than about 5times the fine fiber average diameters in that outermost layer. Thus, itwould have a thickness of no greater than about 25 microns maximum, andin typical applications wherein smaller diameters than 5 microns areused, a substantially smaller thickness. Preferably this outermost layerof fine fibers is relatively permeable having, on its own, apermeability for air of at least 90 meter/min. Of course, if thepermeability of this fine fiber layer is measured in association with acoarse supporting substrate, if the overall combination has apermeability of at least 90 meter/min, the fine fiber layer itself does.

Preferably in this construction there is a layer of permeable coarsefibrous media positioned between each layer of fine fiber media.Preferably each layer of permeable coarse fibrous media comprises fibersof at least 10 microns in diameter and preferably each layer has anefficiency, if evaluated separately from the construction, of no greaterthan 10%, for 0.78μ particles as defined.

Preferably this media construction includes at least three layers offine fiber material, although the at least two layers downstream fromthe "outermost" layer need not necessarily have an average diametersmaller than 5 microns, but rather it would be preferred that they areat least smaller than 8 microns; and, they may be less permeable thanthe outermost layer of fine fiber material, preferably each having apermeability on its own of at least 45 meter/min.

Additionally, a preferred filter media construction according to thepresent invention may be defined as having a first layer of permeablecoarse fibrous media comprising coarse fibers having an average diameterof at least 10 microns, an efficiency of no greater than about 5%, for0.78μ particles, and a first surface on which is positioned a firstlayer of fine fiber media. Preferably the first layer of fine fibermaterial comprises fibers having an average diameter of no greater thanabout 5 microns, and a thickness of no greater than about 5 times theaverage diameter of the fine fibers in this first layer. Preferably thismaterial has a permeability, on its own of at least about 90 meter/min.This media construction, of course, can be utilized in association withother layers of fine fiber and coarse fiber material, and may even beutilized in overall media constructions that use other types of media,for example in association with paper or glass media or other types ofdepth media. The media construction of this embodiment may also includea plurality of further layers of fine fiber material, each of which isspaced from the next adjacent one by a layer of coarse media.

An overall filter construction may be provided, using media according tothe present invention, and as defined in either of the above twoidentified preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a cross section of a theoreticalmono-layer fine fiber filter media.

FIG. 2 is a schematic representation of a cross section of a theoreticalmono-layer coarse fiber filter media.

FIG. 3 is a schematic representation of a cross section of a theoreticalmono-layer fine fiber filter media; FIG. 3 being of a different mediathan that shown in FIG. 1.

FIG. 4 is a schematic representation of a cross section of a theoreticalmono-layer coarse fiber media arrangement having the same percentsolidity as the arrangement shown in FIG. 3.

FIG. 5 is a schematic fragmentary plane view of a surface of a mediaconstruction according to the present invention.

FIG. 6 is a schematic cross sectional view of a media according to FIG.5.

FIG. 7 is a schematic fragmentary cross sectional view of a multi-layermedia construction according to the present invention.

FIG. 8A is a fragmentary schematic perspective view of a pleated mediaarrangement including a media construction according to the presentinvention.

FIG. 8B is an enlarged fragmentary schematic cross-sectional view of aportion of the arrangement shown in FIG. 8A.

FIG. 9 is a schematic representation of a media according to the presentinvention threaded on a mechanical support structure.

FIG. 10 is a side elevational view of a filter arrangement incorporatinga filter media construction according to the present invention therein.

FIG. 11 is an enlarged fragmentary schematic cross sectional view takengenerally along line 11--11 of FIG. 10.

FIG. 12 is a scanning electron micrograph of a conventional air-laidpolymeric fiber media.

FIG. 13 is a scanning electron micrograph of a conventional air-laidglass fiber media.

FIG. 14 is a scanning electron micrograph of a conventional two-phasemedia.

FIG. 15 is a scanning electron micrograph of the same conventionaltwo-phase wet-laid glass media as shown in FIG. 14;

FIG. 15 being taken of an opposite side of the media from that shown inFIG. 14.

FIG. 16 is a scanning electron micrograph of a media according to afirst embodiment of the present invention.

FIG. 17 is a scanning electron micrograph of a media according to asecond embodiment of the present invention.

FIG. 18 is a scanning electron micrograph of a media according to athird embodiment of the present invention.

FIG. 19 is a scanning electron micrograph of a media according to afourth embodiment of the present invention.

FIG. 20 is a scanning electron micrograph of a media according to afifth embodiment of the present invention.

FIG. 21 is a scanning electron micrograph of the media of FIG. 19, afterNaCl loading according to a description herein.

FIG. 22 is a plot of data from Experiment 5.

FIG. 23 is a plot of certain data from Experiment 6.

FIG. 23A is another plot of data from Experiment 6.

FIG. 24 is a scanning electron micrograph of a media according to thepresent invention shown after NaCl loading.

FIG. 25 is a schematic of a custom salt bench used in certainexperiments.

DETAILED DESCRIPTION

A. Filtration Advantages of Fine Fibers

In general, in filter media constructions, some filtration advantagesare theoretically provided by utilizing relatively fine fibers insteadof coarse fibers, for the media. Consider for example FIGS. 1 and 2.FIG. 2 is a schematic illustrating a "single" or "mono-" layer of finefiber media, with a fixed interfiber distance, D_(x), representing thedistance between the surfaces of adjacent fibers. FIG. 2 is a schematicrepresentation depicting a single layer with the same D_(x) but whereinthe fiber diameter is about 12 times larger than the fiber diameter inFIG. 1.

Comparing FIGS. 1 and 2, it is apparent that, for an area of fixed mediaperimeter (i.e. area) the total amount of air space or void spacebetween the fibers in the arrangement of FIG. 2 is substantially smallerthan the void space in the arrangement of FIG. 1. Thus, in thearrangement of FIG. 2, there is significantly less volume available forloading of particulate material trapped by the system. In addition, airflow is more disrupted by the arrangement of FIG. 2, than it is in thearrangement of FIG. 1, since a smaller percent of the surface is openfor undisrupted air flow therethrough.

From a comparison of FIGS. 1 and 2 it is apparent that if averageinterfiber distance (D_(x)) is maintained constant, but average fibersize is reduced, typically a greater space available for loading resultsand higher permeability to air flow results.

Now consider the arrangements of FIGS. 3 and 4. FIGS. 3 and 4 areintended to schematically represent a single layer of fibers in twodepth media systems in which fibers of different sizes are used, butpercent solidity or density is held constant. From a review of thefigures, it should be apparent that the arrangement with the largerfibers, i.e. the arrangement of FIG. 4, has potentially such large openareas that the filter efficiency is relatively low (but permeability isvery high), by comparison to an arrangement with smaller diameter fibersbut the same percent solidity, i.e. the arrangement of FIG. 3.

Theoretical considerations of the effects of utilizing smaller fiberdiameters have been studied and have been represented quantitatively bythe Stokes Number and Interception Parameter.

The dimensionless Stokes Number is represented by the following formula:

    STOKES NUMBER=d.sub.P.sup.2 ρ.sub.P v/9d.sub.f μ

wherein: d_(f) =fiber size (diameter), d_(p) =particle size (diameter),ρ_(p) =particle density; v=approach velocity and μ=fluid viscosity.

From the formula it will be apparent that (at least theoretically) asd_(f) (fiber size) is decreased, Stokes Number is increased (assuming nochange in the other variables).

In general, the Stokes Number is reflective of inertial impaction. Thiscan be understood by considering the likelihood that as an airstream isdistorted or curved around a fiber, a particle within the airstream anddirected toward the fiber will leave the airflow (rather than curve withthe air flow) and impact the fiber. The variables reflected in theformula above for the Stokes Number logically reflect that, in general,an increase in momentum of the particle (from increasing density and/orvelocity) is associated with a greater likelihood that the particle willnot flow around the fiber with the airflow stream, but rather that itwill leave the airflow stream and directly impact the fiber. The formulaalso indicates that this likelihood is greater when the fiber diameteris smaller, due, at least in part, to the fact that when the fiberdiameter is smaller, the fiber will disrupt the airflow stream to alesser extent. This brings the effected flow field of the airstream, asit curves around the fiber, into closer proximity to the surface of thefiber and increases the likelihood that a lower momentum particle willstill leave the air stream sufficiently to encounter (impact) the fiber.

Another consideration relating to why certain fine fiber systems aretheoretically generally more efficient as filters than coarse fibersystems, is particle interception, reflected by the InterceptionParameter. Interception Parameter (R) can be represented by thefollowing formula:

    R=d.sub.p /d.sub.f

wherein d_(p) and d_(f) are defined as above.

In general, Interception Parameter is velocity and momentum independent,and relates to the size of the particle and the size of the fiber. Ingeneral, it relates to the likelihood that a particle (which tends tocurve with the airstream, as the airstream is distorted around thesurface of the fiber), will nevertheless encounter the fiber and becometrapped. Thus, it does not directly relate to the likelihood that themomentum of the particle will carry it out of the airstream and into thefiber, but rather whether, while within the airflow stream, the particlewill nevertheless encounter the fiber. In general, since smaller fibersdisrupt the airflow to a lesser extent, and the distortion in the airflow (from linear) occurs closer to the surface of the fiber, smallerfibers are associated with higher efficiencies and a higher rate ofinterception impactions than larger fibers.

In general, the advantages associated with the use of fine fibers in amedia are more pronounced with relatively small particles. Thus theadvantages of fine fibers may be of particular interest when the filterapplication will require filtering of small particles, especially those10 microns or less in size (diameter).

B. Some Problems and Limits Associated with Utilization of RelativelyFine Fibers in Filter Media

In the previous section, theoretical advantages available from selectionof small diameter fibers in a filter media, relative to coarser fibers,were provided. Problems would result, however, if coarse fibers, i.e. onthe order of about 10 or 12 microns (diameter) on up, were simplyreplaced in depth media by very fine fibers, i.e. on the order of about8 microns and below, typically 5 microns and below, especially on theorder of about 0.2-3.0 microns. For example, constructions made fromfibers on the order of about 0.2-5 microns in size would be moredifficult to handle (than constructions of coarser fibers) and wouldtend to collapse in use, creating a very low permeability. That is, itis relatively difficult to maintain a substantially open structure forhigh loading and high flow therethrough, with a construction merelycomprising fibers of 5 microns or below in diameter, since such mediatypically possesses insufficient mechanical strength (or "body") toresist collapse. When the media collapses, the spaces between the fibersbecome relatively small, and the construction, while perhaps quiteefficient as a filter, loads fairly rapidly and is not very permeable.Indeed, such a system will begin to approximate a surface-loading systemin behavior, since a relatively low porosity and shallow depth is, ineffect, what results.

One can conceive of a construction in which extremely fine fibers areintimately mixed with (i.e. are entangled with) coarse fibers. However,construction of effective filter arrangements, especially usingconventional techniques for creating depth media of mixed fiberdiameters, is not readily achieved when the diameters of the fibers aregreatly different. For example, consider a theoretical system in whichthe fine fibers are 1/20th of the diameter of the coarse fibers. If thefilter media, that the air being filtered passes through, comprises 50%by weight of the coarse fiber and 50% by weight of the fine fiber, thesystem is one in which there is a very high number of fine fibersrelative to coarse fibers (or fine fiber length relative to coarse fiberlength). This would be a system with a relatively low interfiber spacingor porosity. It might be relatively efficient, but it would still loadfairly rapidly. In general, if the weight of the coarse fibers relativeto the fine fibers is reduced, the problem is exacerbated. If the weightof the coarse fibers relative to the fine fibers is increased, theadvantages associated with fine fibers and related to interception andinertial impaction would be compromised.

C. Some Conventional Uses of Fine Fibers in Media

There has been some conventional use of fine fibers in media. Inparticular, Donaldson Company Inc. of Bloomington Min., the Assignee ofthe present invention, has utilized fine fiber technology in itsUltra-Web® products. These products have generally comprised surfaceloading cellulose media, which has a web or net of polymericmicrofibers, of less than 1 micron in diameter, on an upstream surface.

Such media has typically found use in pulse cleaned dust collectors. Inoperation, and without the fine fibers, the coarse, surface loading,cellulose media operates in part as an internal trap for smallparticles. When this occurs, pulse cleaning is less effective since someparticles are trapped inside the cellulose media. However, the fine web,when used, generally operates to collect particulates upstream of thesurface loading cellulose fibers, facilitating particle release when thereverse pulse is applied.

Another use of fine fibers is described in U.S. Pat. No. 4,011,067,incorporated herein by reference. In this reference, fine fibers areapplied to a porous base.

D. A General Approach to Effective Utilization of Fine Fiber Media forFiltration Purposes in a Filter Construction

A general approach for the utilization of fine fibers, i.e. on the orderof 8 or 10 microns or less in diameter, preferably 5 microns or less andtypically about 0.1 to 3.0 microns in diameter (average), in filtermedia has been developed. In general, a very porous, permeable substrateof relatively coarse fibers is used as a support, for the very finefiber media. The material can then be configured in a preferred manner,to achieve an efficient, relatively long life, filter.

In preferred systems, multiple layers of fine fiber media, separated asdescribed, are used to advantage. When multiple layers of fine fibermedia, separated as described, are used, long-life, highly efficientfiltering systems can be readily obtained.

In FIG. 5, a schematic representation of a filter material according tothe present invention is provided. Referring to FIG. 5, the material 1includes coarse fibers 2 and fine fibers 3. The material 1, however, isnot a material in which the fibers of different sizes are mixed togetheror are intimately entangled, when the material is created. Rather,material 1 generally comprises a layer (having depth) of coarse fibers2, at least one outer surface of which has had the fine fibers 3 appliedthereto. That is, the media comprises a web of fine fibers on at leastone outer surface of a structure of coarse fibers. The fine fibers inthe web of fine fibers, then, are not mixed in or entangled with thecoarse fiber support. Herein, the layer of coarse fibers 2 is sometimesreferred to as a layer of permeable coarse fibrous media. It comprises asubstrate on which the fine fibers 3 are positioned.

The arrangement of FIG. 5 can be visualized as somewhat analogous to aspider web strung between the rails of a fence. (The analogy is mostappropriate if it is also assumed that the spider web is positioned andattached at one side or surface of the fence.) The rails, or coarsefibers 2, provide for a very porous open area, and do not substantiallyinterfere with the airflow through the open space. The fine fibers 3represent the web suspended in or across the open space. Since themajority of the airflow stream through such a material is notsubstantially disrupted by the coarse fibers 2, the involvement of thecoarse fibers 2 in interception impaction and inertial impaction isrelatively small. The extremely fine fibers 3, however, are strungacross the volume where the substantial airflow will occur. Advantage isthus taken of the fine fiber size with respect to inertial impaction andinterception impaction. In applications wherein more than one layer offine fiber is used, the spiderweb analogy would involve a plurality offences stacked against one another, each of which had a spiderweb on aside thereof. The effect would be the stacking of spaced-apart finespiderwebs.

In general, FIG. 6 is a fragmentary cross sectional view of materialsuch as that illustrated in FIG. 5. FIG. 6 is schematic in nature. Itwill be understood that, in general, FIG. 5 is greatly enlarged relativeto FIG. 6, so that detail can be understood.

In FIG. 6, the layer of depth media of coarse fibers is representedgenerally at 4 and the layer of very fine fibers is illustrated at 5. Itwill be understood that the fine fibers 5 are applied to surface 6 ofcoarse fibers 4.

In general, for preferred constructions the layer of fine fibers will beconfigured approximately as a mono-layer, and to not have a thicknessmuch greater than about 1-8 fine fiber diameters. In general, its depthwill be no greater than about 10-15 microns at any given location, andtypically no greater than about 2-4 microns.

The depth of the coarse support media 4 will be varied from system tosystem. The schematic of FIG. 6 is simply presented to indicate that ingeneral the depth of the coarse media 4 will be relatively great, bycomparison of the depth of the layer of fine fibers 5.

The construction of FIGS. 5 and 6, however, would be expected to be arelatively inefficient filter, especially if a very open layer of finefibers is used, since a substantially large void volume or interfiberspacing (i.e. spacing between the fine fibers) is provided. That is, airpassing through the void volume and not near a fine fiber would not befiltered to a significant extent, when the fine fibers are sparselydistributed.

As indicated above, according to the present invention, in preferredfilter constructions, material such as that reflected in the schematicof FIG. 5 is arranged in multiple layers, for example, in a stack. Astack of layers, each of which is similar to FIG. 5, would present therelatively fine fibers 3 in a substantially effective density, withrespect to likelihood of being encountered by particles in an airflowstream passing through the entire system. The relatively large voidvolume provided by the very porous coarse fibers 2, would allow for asubstantial loading volume, permeability and thus a relatively longlifetime. It can be theorized, therefore, that such a construction canbe developed which would be both very efficient as a filter and ofrelatively long usable lifetime. As the experiments below indicate, inpractice this is achieved.

A further advantageous aspect of the arrangement shown in FIG. 5 can beunderstood by considering the effect of such a composite on trappingparticles, in operation. In general, if the arrangement is examinedafter a period of particle loading, the particles will appear primarilytrapped on, and secured to, the individual fine fibers, as smallindividual particles or agglomerations of particles. The fiber spacingbetween the fine fibers is sufficiently large that a substantial amountof bridging between the fibers does not occur. This is shown in FIG. 21,discussed below. Indeed the fiber spacing is sufficiently large so thatas bridging begins to occur, the particle dendrites tend to break up andfall through the layer of fine fiber.

This is highly advantageous. In arrangements wherein the fiber spacingis relatively low, but efficiency is relatively high, a substantialamount of particle bridging among the fibers or across the spaces tendsto occur. This can blind off a portion of the filter media involved, tothe passage of air therethrough, and significantly reduce permeabilityof the filter. This leads to an increase in pressure differential acrossthe filter and, eventually, to shorter filter lifetime. Large fiberspacing in a layer, however, reduces the likelihood of this.

As will be understood from further descriptions, the fact that aparticle bridge, if it begins to form, would tend to break and fallthrough the layer or material does not pose a problem with respect toefficiency, since the typical use of material such as shown in FIG. 5will be in arrangements that involve more than one layer of filtermedia.

In this section, certain arrangements according to the present inventioninvolving a stack of media as described in claim 5, were presented. Aswill be expressed in detail in other sections below, a stack can beconstructed by alternately applying coarse and fine fibers to astructure, rather than by combining pre-formed composites (or layers) ofthe type shown in FIG. 5. In the end, the effects on filtering shouldgenerally be the same, however one or the other type of process may bepreferred for reasons not related to the performance of the finalstructure.

E. Typical Constructions

From the above it will be apparent that many typical filter mediaconstructions according to the present invention, when configured foruse to filter, will include multiple layers of media, with at least twolayers effectively comprising a coarse framework supporting fine fibersor a fine fiber web. An example of such an arrangement, sometimesreferred to herein as a stack, is shown schematically in FIG. 7.

In general, stacked arrangements may be constructed from multiple layersof the same media composite. Alternatively, a gradient can be providedin the stacked arrangement, for example, by using somewhat differentcomposite materials in each layer or applying layers appropriately whilemaking the multi-layer composite. The materials in the various layers,for example, may be varied with respect to the average populationdensity of the fine fibers across the open spaces of the coarse support.Alternatively, or in addition, the diameters of the fine fibers can bevaried from layer to layer. Of course, arrangements can include one ormore layers of one particular construction, and also one or more layersof a different construction or more than one different construction.

Referring to FIG. 7, in general a media construction 10 comprising astack of various layers of filter media is shown. For the arrangementshown in FIG. 7, consider an air flow in the direction indicatedgenerally by arrows 11. Construction 10 includes a layer or region 13 ofmedia comprising a coarse support 14 having a thin layer 15 of finefibers on a surface thereof. In the particular arrangement shown, layer15 is on an upstream surface of support 14. Downstream from layer 13 isa similar layer 17 comprising a coarse support 18 and an upstream, thin,fine fiber layer 19. Arrangement 10 includes further layers 20, 21 and22 analogously constructed to layers 13 and 17. Thus, for thearrangement shown in FIG. 7, coarse region 25 of layer 22 is positionedmost downstream.

For the particular arrangement depicted, upstream of the most upstreamfine fiber layer 15, is located a layer 27 of coarse fiber, protective,scrim.

In general it will be understood that for certain embodiments, theoverall construction 10 of FIG. 7 will only be about 0.020 to 0.060 inch(0.05-0.15 cm) thick. Thus, it is enlarged and exaggerated greatly inthe figure. It comprises a stack of layers of fine fibers, each of whichis spaced from the next adjacent fine fiber layer by a coarse separatingor support layer. On each side, i.e. the most upstream side 27 and mostdownstream side 25, is located a protective layer of coarse scrim. Theparticular arrangement of FIG. 7 is shown with five, discrete, finefiber layers, but alternate amounts or numbers of layers can be used.Again, there is no requirement that the fine fiber layers be identicalto one another, or that the various coarse support layers be identicalto one another. By "discrete" in this context it is meant that each finefiber layer is not substantially entangled with the separating coarsesupport fibers, but rather each fine fiber layer generally sits on asurface of a support structure.

1. The Coarse Support/Spacing Structure

A principal function of the coarse material in filter media layersaccording to the present invention is to provide for a framework acrosswhich the fine fibers are extended. Another principal function of thecoarse material is to provide for spacing between the regions or layersof fine fibers, in the stack, so that the separated layers of finefibers do not collapse into a relatively dense (i.e. low permeabilityand relatively low loading) construction. The coarse support/spacingstructure is not typically provided to serve any substantial filteringfunction. Indeed, it preferably is a material so open and permeable thatit does not serve any substantial filtering function.

In general, for typical applications such as those described herein, itwill be preferred that the overall composite (i.e. the resultingmulti-layered, filter media) be a relatively flexible arrangement, whichcan be arranged in a variety of geometric configurations. In somearrangements it will be preferred that the coarse support comprise aflexible fiber construction that has sufficient mechanical integrity or"body" to allow for this. However in some arrangements this "body" canbe provided by a component other than the same coarse fiber materialused to space the fine fiber layers, or it can be provided by theoverall composite. This will be described below.

Some of the more important parameters which relate to the selection ofthe flexible fiber construction for the coarse support, may besummarized by the following:

a. It is preferred to select a material which has a very low percentagesolidity and a very high permeability, if possible, to enhance the "voidspace" across which the fine fiber web will extend. A material which hasa filtering efficiency of only about 10% or less, typically 5% or lessand preferably only 1-4%, for trapping 0.78 micron particles accordingto the test described herein, sometimes referred to as LEFS efficiency,will be preferred. Preferably it is a material having a single layerpermeability when evaluated by the Frazier Perm Test, of at least 150meters/min, typically at least about 200-450 meters/min.

b. The coarse support/spacing material should be sufficiently thick tokeep the layers of fine fibers separated. In general, for some systemsthe layer of coarse material need not be any thicker than is minimallynecessary to achieve this spacing. It is foreseen that a thickness onthe order of about 0.001 inch (25 microns) or so will be more thansufficient. While the material or process selected for the coarsesubstrate may be thicker than about 0.001 inch, for example on the orderof about 0.010 inch (254 microns), the additional thickness is notnecessarily associated with any advantage in connection with actualperformance of the stacked arrangement as an efficient filter. That is,especially in stacked arrangements, thicknesses on the order of 0.001inch (25 microns) will be sufficient to support the fine fibers and canprovide for an open volume for loading of particulates. In many systems,greater thicknesses will not really enhance this operation to anysignificant extent. Thus, in certain preferred arrangements each layerof coarse fiber material, which separates layers of fine fibers, is nogreater than about 0.030 inches (760 microns) thick. Alternately stated,the fine fiber layers are preferably no greater than about 0.03 inches(760) microns) apart. Greater thicknesses, however, are permissible andcan provide for a wider selection of available materials to be used asthe coarse layer. In addition, thicker layers of scrim or coarse fibersmay provide for improvement in "body" or mechanical strength. On theother hand, relatively thick layers may take up an undue or undesirableamount of space in some filter constructions.

c. While the particular material from which the fibers of the coarsesupport are constructed is not critical, in general it will be preferredto select material that is sufficiently strong and tough to withstandmanipulations during manufacture and handling, and also to surviveoperating conditions. It is an advantage of constructions according tothe present invention that the media for many effective filter systemscan be provided without the use of "electrically charged" or "staticallycharged" fibers. Thus, certain preferred systems according to thepresent invention use fibers without static charge applied to them. Inaddition, it is an advantage that the coarse support can be providedfrom readily available fibrous material such as polymeric fibers. Thus,commercially available materials can be chosen as the coarse support orscrim.

d. The material from which the coarse support is formed should be one towhich the fine fibers can be readily and conveniently applied.

While the size of the fine fibers will be selected at least in partdepending upon the particular use for the construction intended, thediameter of the coarse fibers is less important to preferred filteroperation, provided the minimal properties described herein areobtained. In general, it is foreseen that in typical and preferredapplications the fiber diameters of the coarse fibers will be at leastabout 6 times, and typically and preferably about 20-200 times, thefiber diameters of the fine fibers. In typical arrangements wherein thefine fibers have a size of about 0.2-3.0 microns, it is foreseen thatthe coarse material will comprise a fibrous material having an averagediameter of about 10 to 40 microns, and typically 12 microns or larger.The coarse material will typically have a basis weight within the rangeof 6.0 to 45.0 g/m², for preferred arrangements.

In general, the coarse fiber layer may comprise a collection or mix ofshort fibers or a non-woven substantially continuous fiber matrix. Inthis context, the term "continuous" means fibers having an aspect ratiowhich is sufficiently large to essentially be infinite, i.e. at least500 or above. Wet-laid materials may be utilized for the non-wovensupport; however, air-laid also may be used in some systems.

In general, it is believed that commercially available fibrous scrimscan be used as the coarse support. One such scrim is Reemay 2011,commercially available from Reemay Co. of Old Hickory, Ind. 37138. Ingeneral, it comprises 0.7 oz., spunbonded polyester.

Alternatively, Veratec grade 9408353, spun bonded polypropylenematerial, from Veratec, Walpole, Mass. 02081, is usable.

The coarse support layer can comprise a mixture of fibers of differentmaterials, lengths and/or diameters.

2. The Fine Fiber Network or Web

It is foreseen that a wide variety of materials may be selected as thematerial from which the fine fiber web or network is provided. Thefollowing general principles apply to its selection.

a. It should be a material that can be readily formed into fibers withthe relatively small diameter selected, for application to the coarsesupport, or into a web or network of such fine fibers.

b. It should be a material which is sufficiently strong to remain intactduring handling and during the filtering operation.

c. It should be a material which can be readily applied to the coarsesupport.

Hereinabove, reference was made to certain products prepared and sold byDonaldson Company, the assignee of the present invention, under thetrade designation Ultra-Web®. These products comprise a fine fiber webapplied to a cellulose surface media. The process used for thegeneration of these fine fiber webs, in the Ultra-Web® products, is aproprietary trade secret of Donaldson Company. It is foreseen, however,that similar techniques and webs, applied to coarse support structuresas described herein, and used in stacked arrangements as describedherein, would comprise appropriate and useable applications of thepresent invention. This will be made more apparent by the examplesbelow. Other types of fibers and processes can just as effectively beused, however.

In general, for typical constructions according to the presentinvention, it is foreseen that the fine fiber component will be providedwith fiber diameters of 8 microns or less, typically less than 5.0microns, and preferably about 0.1-3.0 microns depending upon theparticular arrangement chosen. A variety of filter materials can bereadily provided in such diameters including, for example: glass fibers;polypropylene fibers; PVC fibers; and, polyamide fibers.

More generally, polyacrylonitrile can be used; polyvinyladine chlorideavailable from Dow Chemicals, Midland, Mich. as Seran F-150 can be used.Other suitable synthetic polymeric fibers can be used to make very finefibers including polysulfone, sulfonated polysulfone, polyimid,polyvinylidine fluoride, polyvinyl chloride, chlorinated polyvinylchloride, polycarbonate, nylon, aromatic nylons, cellulose esters,aerolate, polystyrene, polyvinyl butyryl, and copolymers of thesevarious polymers.

The fine fibers can be secured to the coarse support in a variety ofmanners. The technique used may depend, in part, on the process used formaking the fine fibers or web, and the material(s) from which the finefibers and coarse fibers are formed. For example, the fine fibers can besecured to the coarse support by an adhesive or they may be thermallyfused to the coarse fibers. Coarse bicomponent fibers with a meltablesheath could be used to thermally bond the fine fibers to the coarsefibers. Solvent bonding may be used, thermal binder fiber techniques maybe applicable, and autogenous adhesion may be used. For adhesives,wet-laid water soluble or solvent based resin systems can be used.Urethane sprays, hot melt sprays, or hot melt sheets may be used in somesystems. In some instances, it is foreseen that adhesives for positivesecurement of the fine fiber web to the core support, will not beneeded. These will at least include systems in which, when the overallcomposition is made, the fine fiber is secured between layers of coarsematerial, and this positioning between the two coarse layers is used tosecure the fine fiber layer or web in place.

Herein reference is made to the fine fiber layer comprising "finefibers" or a "network or web" of fine fibers. The term "network" or"web" of fine fibers in this context is meant to not only refer to amaterial comprising individual fine fibers, but also to a web or networkwherein the material comprises fine fibers which join or intersect oneanother at nodes or intersections. An example of such an arrangement isshown in FIG. 20, discussed in greater detail below. From a review ofthe FIG., it can be seen that the network of fine material generallycomprises a plurality of very fine fibers or strands, some of whichextend from nodes or points of intersection.

F. Some Manners of Characterizing a Layer of Media Used in ConstructionsAccording to the Present Invention

In general, from the above it will be apparent that a layer of mediaused in constructions according to the present invention will generallyinclude a coarse support having a layer or web of fine fibers secured toat least one surface thereof. The coarse support and fine fibers may begenerally as previously described. The overall layer may becharacterized in a variety of manners, including, for example, simply ascomprising coarse and fine fibers as described and also arranged asshown.

It is not accurate to characterize media according to the presentinvention as comprising a "mixture" of the fine fibers with the coarsefibers. The material is not generally constructed as a mixture of suchfibers, i.e., an arrangement wherein the fibers are entangled. Ratherthe fibers are located, within the media, in separate and discrete zonesor regions. More specifically, any given one of the composite layersgenerally comprises the layer of coarse material having at least onesurface on which is applied the fine material. Even when the media isprovided in multilayer (stacked) arrangements, the regions of fine fiberand coarse fiber are generally separately encountered as air passesthrough the "stack".

As will be apparent from the overall description herein, a variety ofmethods can be utilized to prepare stacked arrangements according to thepresent invention. In some, for example when the layers are wet-laid toachieve the embodiment, there may be some entanglement of the fine andthe coarse fibers. The degree of entanglement would of course not be tosuch an extent that the fine and coarse fibers would be a "homogenousmix" or the media would not perform desirably according to theprinciples of the present invention. In general the coarse layers wouldstill be used to separate the various fine fiber layers from oneanother, in the arrangement. Herein, when the fine fiber layers aredescribed as "discrete" relative to one another and relative to thecoarse fiber layers, it is not meant that there is absolutely noentanglement, but rather the construction is such that the multi-layer,i.e. separated fine fiber layer, environment is provided for filtration,as the fluid to be filtered passes through the arrangement. In generalthis will mean that such entanglement that may occur is relatively low.Generally the entanglement between the fine fiber layers and coarsefiber layers, if it occurs at all, will only involve a relatively smallpercent by weight of the fine fibers, typically less than 15%.

As a result of possessing the structure described in the previousparagraph, a homogenous filter media is not presented to the air flow.That is, as the air passes through the filter arrangement, at variousdepths or levels, different materials are encountered. For example, insome systems the air would pass through alternating rows of fine fibermaterial and coarse material, as it passes through the system. Importantadvantages result from this.

In typical arrangements, the composite layer of media may becharacterized with respect to the mass of fine fiber applied per unitarea of a surface of the coarse support or scrim. This is sometimesreferred to as the basis weight of the fine fiber layer. Such acharacterization will be varied depending upon the particular fiberdiameter used, the particular material chosen and the fiber diameter andthe particular fine fiber population density or filter efficiencydesired for the layer. It is foreseen that in typical, preferredconstructions having fine fiber diameters of about 0.1 to 5.0 microns,the mass of material from which the fine fibers are formed, applied perunit surface area of scrim or coarse support, will be within the rangeof about 0.2 to 25 g/m², regardless of the particular material used.

An alternate method to characterize a typical and preferred media layerin constructions according to the present invention is with respect tothe amount of interfiber space open or visible, when looking into thecoarse fiber support or scrim (from the fine fiber side), that isoccupied by or covered the fine fibers or web of fine fibers. Thismethod of characterization will be understood, in part, fromconsideration of FIGS. 16-20.

FIGS. 16-20 are scanning electron micrographs, at variousmagnifications, of various examples of scrim with a fine fiber webaccording to the present invention on one surface thereof. The coarsesupport comprises polyester fibers of 25 to 35 microns in diameter. Thefine fibers generally comprise glass fibers from about 0.1 to 3 micronsin diameter.

The percentage of the area of the open pores in the scrim, occupied bythe fine fibers, by area, can be estimated from evaluation of SEMs suchas that depicted in FIGS. 16-20. It is foreseen that for typical andpreferred constructions according to the present invention, the averagepercentage of the open area in the coarse support or scrim occupied bythe fine fibers, when evaluated using such a method, will be 55% orless, typically about 20 to 40% for preferred air filter media. It isnot meant to be suggested that constructions outside of these rangeswill be inoperative, but rather that such percentages are typical andare associated with generally operable and effective materials.

Another manner in which one can characterize some layers of materialaccording to the present invention, arrangeable for use in a filterconstruction as described, is with respect to observations aboutperformance, when the material is tested in an air flow stream. Inparticular, in general after testing in an air flow stream forefficiency as described herein, it will be observed that a majority ofparticles (both by number and by mass) stopped by the layer will beengaged by the fine fibers, rather than the more coarse construction, inmany preferred embodiments. An example of this is shown in the electronmicrograph of FIG. 21.

Alternatively, or analogously, the efficiency of any given compositelayer can be assigned, based upon empirical observation. In general, ifa coarse fiber support structure comprising fibers having an averagediameter of at least 10 microns, and also having an efficiency of 6% orless, for 0.78μ particles when evaluated as described herein, isimproved by application of at least one fine fiber layer thereon,wherein the fine fibers have an average fiber diameter of about 5microns or less, such that the improved material when tested has anefficiency of at least about 8%, and preferably at least 10%, for the0.78μ particles defined, the construction will be one which has at leastsome of the desirable properties for use in at least certain preferredarrangements according to the present invention. Preferably, thematerial comprises a scrim having an efficiency of about 4% or less for0.78μ particles, to which sufficient fine fibers have been applied toprovide a composite efficiency of at least 10% or more for 0.78 micronparticles. In such arrangements, preferably the coarse fiber scrim is amaterial having a permeability, without the fine fiber layer appliedthereto, of 250-450 meters/min. Also, typically the fine fiber materialis arranged such that the permeability of a single composite layer ofthe fine fiber/coarse fiber combination is at least about 10 meters/min,more preferably at least about 25 meters/min. In some instances it maybe chosen to be significantly higher, i.e. 100-325 meters/min.

G. Permeability

Herein reference is made to the permeability of: any given layer ofscrim materials; a composite or layer of scrim with at least one layerof fine fiber thereon; and the overall media composite. In thesecontexts, the numerical references to "permeability" are in reference tothe media face velocity (air) required to induce a 0.50 inch H₂ Orestriction across a flat sheet of the referenced material, media orcomposite.

In general, permeability of a media layer, as the term is used herein,is assessed by Frazier Perm Test according to ASTM D737, using a FrazierPerm Tester available from Frazier Precision Instrument Co., Inc.,Gaithersburg, Md., or by some analogous test.

Typical media arrangements according to the present invention,especially when used in auto cabin air filters, ventilation systems orengine air induction systems, will have an overall permeability of atleast 6 meters/min, and more preferably 10-20 meters/min, withpermeability being a function of the overall efficiency, number oflayers and size of selected fibers. By "overall" in this context,reference is made to the complete media stack.

H. Efficiency

Herein throughout, reference to the efficiency of a layer or composite,in specific numerical terms, is sometimes made. That is, sometimes aselected layer of scrim, or scrim with at least one layer of fine fibermaterial thereon, will be described with respect to a preferredpercentage efficiency. Also, in some instances, numerical percentageefficiencies are described in connection with an overall composite,comprising multiple layers of material. In these contexts, and toprovide the numerical characterizations made, "efficiency" is typicallymeant to refer generally to the percentage of test particles retained,when the material characterized is tested according to the method ofASTM 1215-89, incorporated herein by reference, and wherein the testmaterial applied is 0.78 micron diameter, mono-dispersed, polystyrenelatex spheres, such as those available from Duke Scientific, Palo Alto,Calif., tested at 20 feet/min. (about 6 meters/min).

From the above it is not meant to be suggested that when a filterarrangement according to the present invention is generally described as"efficient", it is merely described with respect to its operation infiltering 0.78 micron particles under the test conditions of ASTM1215-89. Rather, efficiency for such particles and under such testconditions is merely one relatively reproducible manner in whichmaterials used, or to be used, in constructions according to the presentinvention can be evaluated or characterized.

I. Fiber Spacing; Weight of Fine Fiber Applied Per Unit Area of CoarseSubstrate

As indicated above, materials according to the present invention can becharacterized with respect to either fiber spacing or the amount of finefiber material applied per unit area of the coarse substrate or scrim(basis weight). Methods usable to accomplish this are as follows.

1. Area Solidity by Digital Image Analysis

The method employed here is to scan an SEM photo into a computer forimage analysis. Usable SEM magnification depends on the size of thefibers of interest in the media structure and should be selected so theedges of the fibers to be analyzed are distinct from the background. Asmagnification is increased, the depth of the viewing field is reduced.After scanning, one can use commercially available software such asVisilog (from Noesis Vision of Ville St. Laurent, Quebec, Canada) toseparate the image into foreground and background by setting a grayscalethreshold value which defines the border between foreground andbackground, and converting the scanned grayscale image into a binaryimage (foreground and background). A more refined separation of theforeground and background binary image can be achieved through the useof the erode and dilate commands. Items of interest are the fine fibersin the foreground. (Assuming the media to comprise scrim having finefibers applied to only one surface thereof; and, also assuming the SEMorientation being to show the fine fibers toward the viewer.) Onceseparated, screen pixels used to display the foreground and thebackground can be counted using analysis tools within the software. Theratio of the number of pixels used to display the foreground (finefibers) divided by the number of pixels used to display the area ofinterest (AOI=foreground+background) from which the fine fibers weretaken, represents the area solidity. Fibrous structures are3-dimensional, while SEM photos represent a projection of a3-dimensional object onto a plane or area, hence the term "areasolidity".

2. Digital Pore Size Analysis

The method employed here is to scan an SEM photo into a computer forimage analysis, again using commercially available software such asNoesis Visions Visilog. Usable SEM magnification depends on the size ofthe fibers of interest in the media structure and should be selected sothe edges of the fibers to be analyzed are distinct from the background.As magnification is increased, the depth of the viewing field isreduced. After scanning, one can use commercially available softwaresuch as Visilog by Noesis Vision, to separate the image into foregroundand background by setting a grayscale threshold value which defines theborder between foreground and background, and convert the scannedgrayscale image into a binary image (foreground and background). A morerefined separation can be achieved through the use of the erode anddilate commands. Items of interest are the pores created by the finefibers in the foreground. Next, items in the image's AOI which representanomalies to the software's analysis tools need to be removed from theAOI. Such anomalies include convex pores and pores that lie partiallyinside the original AOI, i.e. the borders of such pores are not fullydefined. Software tools can then be used to calculate the perimeter,area, and aspect ratio in pixel dimensions for each cell inside therevised AOI. A shape factor defined as:

    (4π×Pore Area)/(Measured Pore Perimeter).sup.2

for each pore, inside the revised AOI, can be calculated. From thescanner resolution, SEM photo magnification, and software output, onecan convert interfiber spacing dimensions from pixel units toengineering units. The procedure should be repeated sufficiently toensure a representative figure (or distribution) for the sample.

3. Line Fiber Intersection Method

First, SEM photos of media samples of appropriate magnification andnumber to determine the fiber size distribution of the media are taken.For fiber sizing, magnifications typically range from 1,000 to over6,000×. From another set of SEM photos, magnified such that at least 15to 50 pores appear in the photo, a grid of lines can be superimposedonto (a magnified copy of) the SEM. Using randomly selected lines fromthe superimposed grid, the number of fibers intersecting the randomlyselected grid lines can be counted so that the number of intersectionsper inch of line is known. By accumulating data for a statisticallysignificant number of lines, one can calculate average interfiber spacesand devise a distribution of interfiber distances. The procedure shouldbe repeated sufficiently to ensure a representative figure (ordistribution) for the sample.

4. Basis Weight

The fine fiber weight applied per unit area (surface) of coarse support(for example Lbs/3000 ft² or grams/m²) can be used to estimateinterfiber space dimensions since in typical constructions the finefiber mat of interest approximates a monofiber layer. Alternately, ifthe fiber structure is not a mono layer, and the thickness of the fibermat can be determined, then volume solidity can be calculated, which isa fiber spacing index.

5. Pore Size

a. Mitered Cylinder Geometry

Usually, where layers of fibers are in intimate contact, pores areassumed to be square with mitered fiber intersections, and layers arerandomly oriented relative to each other. Ref "Fluid Filtration: GasVolume 1" ASTM Special Technical Publication 975, © 1986, ASTMpublication 04-975001-39, Donald R. Monson--"Key Parameters Used InModeling Pressure Loss Of Fibrous Filter Media".

L=centerline distance between parallel fibers on opposite sides ofassumed square pore.

d_(f) =mean square fiber diameter.

b=L-d_(f) =interior pore size dimension, excluding thickness of fibersframing the pore.

C=solidity=fiber volume/media volume.

R=L/d_(f)

Using the above definitions, consistent units of measurement, and thefollowing equations developed by Monson, the interfiber distance "b" canbe estimated from the following equations:

    R=1/{1.1781-(1.3879-1.5×C).sup.1/2 }

    b=(R-1)×d.sub.f

b. Mitered Cylinder Geometry: Modified

This model corrects for spacing between consecutive layers of fibers,assumes an inter layer distance of L/2, and is considered valid forvalues of C<0.6.

    R=1.4472/(0.5×C).sup.1/2 ×COS{1/3×COS.sup.-1  -0.87979×(0.5×C).sup.1/2!}

    b=(R-1)×d.sub.f

c. Mitered Cylinder Geometry: Modified for Mono Layer Fiber Structure

    R=(0.5×π/C)+({0.5×π/C}.sup.2 -{8/ 3×C!}).sup.1/2

    b=(R-1)×d.sub.f

J. Design of a Filter Media Composite Using Principles According to thePresent Invention

From previous descriptions it will be understood that, generally, filterconstructions using media according to the present invention will beorganized with the media comprising layers, more specifically layers offine fiber separated or spaced apart by the coarse scrim material orcoarse fiber matrix. In many instances of designing an air filterconstruction, then, the engineers will be engaged in a process ofselecting the nature of the individual layers and determining how theyshould be organized in the overall composite. In this section,considerations with respect to this process are briefly discussed.

In general, the selection will depend in part upon the use to which thefilter media is to be applied and how the media is made. The intendeduse will generally result in a definition, for the filter designer, ofthe efficiency of the filter and permeability needed. The efficiency fora given use may be defined by means other than by ability to trap 0.78micron particles under the test conditions provided above. For example,the manufacturer of an automobile may have specific specifications forthe operation of a cabin air filter, which the filter engineer is tomeet using materials according to the present invention. Thatspecification might be defined with respect to the ability to trapparticles under test conditions that are not equivalent to those definedherein with respect to 0.78 micron particles. The engineer could use thetechniques described herein to approximate the possible construction,and then develop appropriate testing to see that the specificationsprovided by the automobile manufacturer are met. As an alternative,after sufficient testing, the engineer may develop sufficientcorrelation data to be able to predict performance under one type ofcondition, based upon tests conducted under another.

In any event, in general it is foreseen that in some instances thedesign process will begin with the engineer considering availablematerials, that possess properties according to the present invention.As an example, the engineer may select a scrim and obtain varioussamples of the scrim material with various amounts of fine fibermaterial applied thereto. As a hypothetical example, assume that theengineer has had various samples of scrim material comprising Reemay2011 treated with fine fiber glass material in various amounts, on onlyone surface thereof, to create eight samples in which the fine fiberlayers are characterized by the following:

    ______________________________________                                        Glass Fiber                                                                   Schuller #106.sup.1                                                           (0.4μ) fiber                                                               Glass                                                                         Wt in                     Area                                                Hand              Single  0.44 ft.sup.2                                       Sheet   Single    Layer   Slurry Glass                                        Former  Layer     LEFS    Weight/Unit Area                                    Slurry  perm      Effic                Lb/                                    (g)     (fpm)     (%)     g/ft.sup.2                                                                           g/m.sup.2                                                                           3000 ft.sup.2                          ______________________________________                                        0.035   818.6     6.7%    0.079  0.848 0.520                                  0.040   686.1     8.8%    0.090  0.969 0.595                                  0.075   282.2     25.5%   0.169  1.816 1.115                                  0.100   189.7     32.6%   0.225  2.422 1.487                                  0.150   123.8     54.5%   0.338  3.633 2.230                                  0.200   88.0      68.8%   0.450  4.844 2.974                                  0.380   33.7      94.3%   0.855  9.203 5.650                                  0.600   21.0      99.1%   1.350  14.531                                                                              8.921                                  ______________________________________                                         .sup.1 Schuller #106 is a glass fiber having a median fiber diameter of       0.4 microns, available from Schuller Filtration of Littleton, CO 80162.  

Given the above-available information and materials, the filter engineercould conduct the designing process. In general, the efficiency of thecomposite for the 0.78 micron particles under the test conditionsdefined, will be the "sum" of the efficiency of each of the layers. Forexample, if two layers are used, each of which is 35% efficient, anoverall efficiency of 1- (1-0.35)×(1-0.35)! or 57.75%. Thus, fromknowledge about the various layers, the engineer is in a position to beable to determine how many layers and which materials to use, in orderto achieve a desired level of efficiency.

In the previous paragraph, a general formulation for determiningefficiency in a multilayer system was presented. The specificcalculation was made according to the following principles:

For a stated particle size and velocity:

η_(i) =layer efficiency for layer i

η_(t) =total composite efficiency

1-η_(i) =layer penetration for layer i

1-η_(t) =total composite penetration (1-η_(t))=(1-η₁)(1-η₂) . . .(1-η_(i))

In general, the overall permeability of the composite can be determinedfrom the permeability of the various layers in the composite accordingto the following mathematical relationship: ##EQU1## wherein:ρ_(composite) =permeability of the total composite

ρ_(i) =permeability of component layer of composite comprising either:layer of coarse+fine; or layer of coarse alone, depending onconstruction.

Thus, with knowledge about permeability of the various layers, theengineer can know what the permeability of the overall composite willbe; and, various layers can be chosen to provide a particular desiredpermeability. As an example, the typical face velocity of a cabin airfilter is 50-70 ft/min (about 15-24 meters/min) and such an arrangementoperates with an air flow of 220-300 ft³ /min (about 6.2-8.5 meters³/min). This can, for example, be achieved with a filter made of thefollowing composite:

1. An upstream layer or matrix that is 30% efficient;

2. A next downstream layer or matrix that is 35% efficient;

3. A next downstream layer or matrix that is 45% efficient.

The composite would then be about 75% efficient.

It can be seen, then, that if the engineer knows: what permeability(under the test conditions used to defined the permeability of the givenlayers) is desired for the overall composite; and, the engineer hasdetermined what the efficiency of the various layers is, and knows whatthe efficiency of the overall composite under similar conditions isdesired to be, the engineer can readily select from among the materialsto achieve the desired results.

The engineer may wish, of course, to take into consideration othervariables or factors. For example, fewer layers may be associated with athinner composite, and in some instances a preferred overallorganization. Cost, availability of materials for any given layer, andother related factors may be of concern. Also, the resulting physicalproperties of the composite, for example with respect to ease offormation of a pleated construction, may be of concern.

As an example of the kinds of considerations that can be taken intoaccount by the filter designer in selecting the materials, consider thefollowing. If two sheets of material of the same area, one of which issubstantially thicker than the other, are pleated, in general the numberof pleats that can be effectively positioned within a given volume willbe greater for the thinner sheet. Thus, if the filter design problem isto create an efficient filter for a given cylindrical space, theengineer may have a preference for a thinner pleated material, relativeto a thicker one. If the pleated material is to be made from layers ofmedia according to the present invention, the engineer may prefer toselect a combination of layers that leads to a relatively thin overallconstruction, as opposed to a thicker one, to advantage.

However, in general in comparing composites of equal overall efficiency,a thicker composite will generally have a longer life (on an equal areabasis) than a thinner one. This factor would be balanced by the engineeragainst the concerns of the previous paragraph, in designing somesystems.

It is foreseen that in some instances the engineer will desire to haveall layers of the stack comprise the same composite material. However,in other instances different materials (or efficiencies etc.) may beused in some or all of the layers. It is foreseen that in typicaloperations, should the engineer determine to have layers of differentefficiency in the composite, in general the resulting efficiencygradient will preferably be arranged such that the efficiency of thecomposite layers generally increases toward the downstream side of theconstruction. That is, it is presently foreseen that the preferredorganization of layers will be such that more efficient composite layersare further downstream than less efficient composite layers, so thatlonger life results. The reason longer life generally results under suchcircumstances is that the higher efficiency layers will not occlude asrapidly if positioned on the downstream side, since the less efficientlayers will have operated to provide at least some filtering before thefluid stream reaches the more efficient layers. This means that the moreefficient layers will tend to occlude less rapidly than they would ifpositioned further upstream.

K. Geometry

Media according to the present invention may be arranged in a widevariety of geometric configurations, to advantage. For example, flatsheets can be arranged in a simple stack to form media for a non-pleatedpanel type filter.

Of course, the sheets can be arranged as a blanket or wrap around anitem, for example as a cylindrical wrap around a cylindrical structure.

Uniquely, a media according to the present invention can be provided ina form such that it can be readily pleated. In some instances, this willbe accomplished by selecting the spacing scrim such that when stacked,the resulting composite has sufficient strength or "body" to be pleatedand to retain the pleated configuration. This is illustratedschematically in FIGS. 8A and 8B.

Referring to FIG. 8A, filter media 30 is depicted in a pleated,cylindrical configuration. In FIG. 8B, a portion of the material isshown in exaggerated blow up, so that it will be understood the materialcomprises a plurality of layers. Referring to FIG. 8B, material 30includes coarse layers 31 with layers 32 of fine fibers positionedtherebetween. In general, it is foreseen that for many preferredarrangements, the number of pleats, whether arranged cylindrically or ina panel, will be about 1 to 15 per inch (or per 2.5 cm). When acylindrical configuration is described, the number of pleats perdistance reported herein is with respect to the inner diameter of thecylindrical construction.

A pleated, cylindrical configuration according to the present inventionis unique at least for the reason that media according to the presentinvention greatly exhibits the properties of depth media systems, withrespect to loading and operational face velocities. However,conventional depth media is not generally pleated. More specifically,pleated constructions are generally associated with paper or cellulosesurface loaded systems. However, the principles of the present inventioncan be utilized to provide an arrangement which operates as a form ofdepth media, but which can be configured in a pleated manner moresimilarly to surface loaded cellulose media.

It is noted that for some arrangements a sinusoidal (weave orpleat-like) arrangement can be provided even though sufficient body isnot provided by the media to retain a truly pleated configuration.Several methods for accomplishing this are foreseeable. In the first,the "body" can be provided by having only one or a few layers ofmaterial (in the media) possess sufficient body. For example, one or twolayers of scrim in a multilayer system can be enough for this bodywithout requiring all of the layers to possess it. In addition oralternatively, support layers of material within, or on one side or bothsides of, the stacked arrangement may be used to provide this body ormechanical integrity. Such a laminate composite can be made usingcommercially available synthetic or cellulosic fibers as the supportlayers.

A second approach to providing a sinusoidal arrangement without havingthe coarse scrim layers form pleats, is to utilize a mechanicalframework to maintain the material in the pleated construction. Aschematic with respect to this is illustrated in FIG. 9. In FIG. 9,mechanical stays 40 are depicted, with media 41 threaded thereon.

Other approaches may involve sufficient thermal, chemical or physicaltreatments of the material to provide sufficient rigidity to retain apleated configuration or corrugation. Pleat tip bonding approaches mayalso be used. Also, metal ribbons or wires positioned within the variouscomposite layers can be used to maintain a pleated configuration.

From the above considerations, it will be apparent that an advantage ofthe present invention is that it may be applied in materials providingfor a wide variety of geometric configurations. Thus it can be appliedin a great many filter constructions to advantage. As indicated above,the properties of the materials may be selected so that the depthneeded, for efficient operation, can be varied as desired.

L. An Air Filter Construction

Media according to the present invention may be utilized in a very widevariety of air filter constructions. It can be used, for example, ascylindrical pleated material in cylindrical elements. It may also beutilized as pleated material in panel-type filters. It can be used inunpleated forms, for example as sleeve filters inside of other filterelements, or around the outside of other filter elements. It can also beused in unpleated form in cylindrical and panel elements. Indeed, it mayfind application to replace the media, or a portion of the media inalmost any of a wide variety of filtration or filter systems.

In some instances, media according to the present invention may beutilized to enhance the operation of other media, for exampleconventional media. Thus, media according to the present invention maybe applied on an upstream side of, a downstream of, or between layers ofconventional media, to achieve preferred filter operation. For example,a high efficiency version of media according to the present inventionmay be used downstream of convention media, as a polish filter. A highload, lower efficiency version of media according to the presentinvention may be utilized on the upstream side of conventional media, toachieve an increase in overall efficiency by utilization as a high loadmedia on the upstream side. Media according to the present invention mayalso be utilized between layers of conventional media, in variousgradient filter systems or related systems.

One type of filter construction according to the present invention isillustrated in FIGS. 10 and 11. The arrangement of FIG. 10 is generallyanalogous to that depicted in U.S. Pat. No. 4,720,292, incorporatedherein by reference, except that the filter media has been replaced withimproved media according to the present invention. Referring to FIG. 10,the filter arrangement 100 depicted includes a housing 101, an outlettube 102, and a filter element 103. Access to an interior 104 of thehousing 101 for maintenance of the filter element 103 is through hatchor cover 105.

The filter element 103 generally comprises pleated filter media 110,outer liner 111, and inner liner 112. For the arrangement shown in FIG.10, air filtration occurs by air passage in the general direction ofarrow 115. Thus, housing 101 includes inlet 120 for air to be filtered.The air is distributed in chamber 121, before it passes through filterelement 103. The air then enters internal chamber or bore 122, and exitsthe filter element throughout outlet member 102.

The filter element 103 includes first and second opposite end caps 130and 131, respectively. The filter media 110 is secured to, embeddedwithin and extends between, the end caps 130 and 131. End cap 130 issized and configured to form a radial seal with outlet tube 102, inregion 140. End cap 131 closes end 142 of the filter element 103, in aconventional manner.

In FIG. 11, a portion of the arrangement shown in FIG. 10 is depicted ina schematic cross sectional view. It can be seen that the filter media110 is a multi-layer arrangement according to the present invention, andcontains a plurality of layers 150 of coarse material, and spaced apartlayers 151 of fine fiber material. The particular arrangement shown inFIG. 10 has two fine fiber layers 151 spaced apart and sandwiched by atotal of three coarse layers 150. Again, according to the principles ofthe present invention, a variety of alternate arrangements may beutilized as the filter media 110.

The media is shown, in FIGS. 10 and 11, incorporated in a cylindricalelement constructed for radial sealing with the outlet tube. The mediamay also be used in filter elements for axial sealing arrangements.

In general, the invention can be used to prepare media having highloading capacity when compared to surface-loading media, but its loadingadvantages are believed to be more pronounced when the operation is forfiltering a fine particulate matter, by comparison to filtering tocollect more coarse matter. Therefore, by pleating the invention andplacing it downstream of some conventional depth media, advantages canbe obtained, since the conventional depth media would collect the coarseparticles relatively efficiently, allowing enhancement of the filtrationprocess due to the high efficiency for fine particular matter of mediaaccording to the present invention. The utilization of depth mediaupstream for more efficient filters is described in U.S. Pat. Nos.5,082,476; 5,238,474; and 5,364,436 and using similar techniques buthaving downstream from the depth media, media according to the presentinvention, advantages can be obtained. For example, depth media such asthose described in the above patents can be used to remove particles,very efficiently, in a size ranges from 2-10 microns upstream from mediaaccording to the present invention, with the media according to thepresent invention used to achieve very high efficiency removal of sub-2micron materials, downstream. Thus, media according to the presentinvention can be used in a complimentary manner, with more conventionaltechniques.

In some applications, media according to the present invention may beconfigured such that it is not pleated, but rather such that it islocated downstream of the depth media and used in either a panel orcylindrical filter element. The media may be a separable component froma remainder to the filter assembly, such as a serviceable replacementpart. The media according to the present invention may also be utilizedupstream above the filter components, as a replacement part. Methods ofusing replaceable media sections are described, for example, in theabove U.S. patents, and also in U.S. application No. 08/426,220,incorporated herein by reference.

M. Positioning of the Fine Fiber on the Coarse Support; Orientation ofthe Fine Fiber Layer with Respect to Fluid Flow

As explained above, generally filter material according to the presentinvention comprises a coarse support with a fine fiber web or matapplied to at least one surface thereof. As should be apparent fromevaluation of the drawings, in general a coarse fiber support can beviewed as having two available surfaces for application of the finefiber, one on each side of the coarse fiber mat. There is no particularreason why a fine fiber mat cannot be applied on opposite surfaces of alayer of coarse material, in at least some useful systems. The coarsematerial would serve to separate the fine fiber mats appropriately. Itis foreseen that if such arrangements were to be utilized in a stackedmanner, in some instances it would be desirable to place a layercomprising only coarse material or scrim between those layers of coarsematerial having the fine fibers on both sides thereon, to maintain aseparation between each layer of fine fibers. However, it is certainlypossible that in some instances the amount of fine fiber applied may beselected such that when placed immediately adjacent to a fine fiberlayer on another coarse layer, a desired total level of efficiencyresults.

As to orientation of the material with respect to air flow, in a stackedarrangement no particular preference is perceived. That is, the finefiber layer may be on either the upstream side or the downstream side,of the mat to which it is applied.

N. Selection of Media for Use in Arrangements According to the PresentInvention

As indicated above, at present it is generally believed that the factorsof most concern regarding the media relate to selecting the materialssuch that the coarse fibers are well spaced, serve relatively littlefiltering function, and are appropriately positioned to support the finefibers and to keep the layers of fine fibers separated from one anotherin the overall construction. The fine fibers are chosen for theirrelatively small diameter. Thus, it is foreseen that a very wide varietyof materials can be utilized in constructions according to the presentinvention, and a wide variety of techniques are applicable to thegeneration of such materials.

In general, techniques for the preparation of fiber materials for use infilter constructions according to the present invention are not withincommon practices of a filter designer or engineer, but rather are in thefield of fiber processing and polymer processing. It is foreseen thatcompanies such as Hollingsworth & Vose, East Walpole, Mass. 03032; andLydall, Inc., Rochester, N.H. 03867 are knowledgeable in the techniquesof generating fine fibrous materials or applying them as layers to othermaterials. Also, as indicated above, Donaldson Company Inc. hasdeveloped some trade secret technology in this area relating to itsUltra-Web® products.

The following references incorporated herein by reference, generallydescribe the generation of fine or ultra-fine fibers: U.S. Pat. Nos.2,450,363 (for glass fibers); No. 4,650,506; Wente, Van A., "SuperfineThermoplastic Fibers," Industrial Engineering Chemistry, Vol. 48, p.1342 (1956); and, Schoffel, Norman B., "Recent Developments withMicroglass Media," Advances In Filtration and Separation Technology,Vol. 9, p. 184-199 (1995). In general, it is foreseen that many of thesematerials could be used as a fine fiber web in constructions accordingto the present invention.

As to the coarse fiber mat, again a very wide variety of materialsincluding at least the commercially available materials described hereinwould be useful.

As to the techniques used to generate the fine fiber web and apply it tothe surface of the coarse material, again various approaches would beuseable, and it is foreseen that the preferred ones for any givenapplication would depend at least in part upon the filter application,processing equipment available, the particular material chosen for thefine fiber web, and the material and process used to manufacture thecoarse fiber substrate. At least such techniques as wet-laid processing;air-laid process; melt-blown processing; and polymeric fiber spinningwould, in general, be useable.

Although no particular approach to fiber generation or materialcomposition is presently foreseen as most preferred for a useful filterconstruction, the inventors have evaluated and compared media made fromtwo approaches. One is the utilization of fine fibers comprising glass,as a possible process for preparing media, applied to Reemay 2011. Ithas been found that the process does generate useable media meeting thegeneral criteria of the present invention. The other is fine polymericfibers generated by using a modification of the trade secret DonaldsonCo. Ultra-Web® process, to apply fine fibers to a substrate of Reemay2011.

In particular, with respect to glass fibers the inventors havesuccessfully utilized an approach based on a variation of thedescription in Example 1 of U.S. Pat. No. 5,336,286, incorporated hereinby reference, for formation of a useable material. Example 1 of the '286patent was modified by using glass or glass fibers for the fibermaterial to be applied, and using as the material on which the finefibers are deposited Reemay 2011 scrim. In addition, 2 ml of HCl acid(37%) were placed in the water/fiber slurry in a kitchen blender, tochemically disperse the glass fibers.

Thus, as a result of the modifications, the wet-laid deposition occurredby placing the glass fibers onto a Reemay 2011 scrim which had beenpositioned above the screen, to form a fiber sheet.

Other samples, made using a variation of the Donaldson Co. trade secretUltra-Web® process, are described below.

No substantial difference in performance for the two types of materialswas observed, in comparative testing below. Thus, with respect to theprinciples of the present invention, there is no basis for a preferencefrom between them.

O. Application of the Techniques Described Herein to Mist Collection

In general, it is foreseen that some of the materials described hereinmay be utilized in constructions for the filtering of very fine mistsfrom the air. In general, such mists comprise droplets of about 1 micronin size or smaller. At this size, to some extent they can be treated asparticles for purposes of evaluating filtration. Certain materials asdescribed herein, then, can be used to trap such mists. In general, highseparation efficiency can be obtained without the small interfiberspaces (i.e. pores) typical of high efficiency mist filter media. Thesmall pores of conventional high efficiency media retain the separatedliquid due to capillary forces. The retained liquid in such systemsrapidly increases buildup of flow resistance to passage of air, whichshortens useful filter life. It is foreseen that for such applications,fiber surfaces which are phobic with respect to the fluid beingcollected can be used to advantage.

P. Liquid Filtration Systems

In general the techniques described herein can be utilized inapplications involving a wide variety of fluid streams. Many of thespecific descriptions provided thus far have been in association withair or gas flow streams, carrying particulates. It is foreseeablehowever that the materials described herein may also be utilized insystems for filtration of liquids. That is, the liquids would bedirected through the media according to the present invention, withparticulates therein trapped as described.

In general, if applied in liquid systems, it is foreseen that theprinciple of increasing filter media life by increasing the number ofspaced fine fiber layers will be substantially the same for liquid asfor air, though the collection mechanism is sieving. Because thecollection mechanism is believed to be sieving, the composite efficiencyof in a liquid application is limited by the efficiency of the singlelayer with the highest efficiency. Typical efficiencies for liquidapplications vary from about 50% for lubrication oil to about 99% forfuel filter media. Therefore practical embodiments of the inventionapplied to liquid filtration systems would in general be comprised oflayers with higher layer efficiencies, than would be utilized in airfiltration systems of lower composite efficiencies such as for gasturbines, engine air induction systems, cabin and indoor air ventilationsystems. The invention may also provide advantages when applied to airfiltration applications with high efficiency requirements such as HEPAgrade filtration in a ventilation system. The invention may provideadvantages in terms of filter life and more reliability through multiplelayers. Through redundant filtration, the overall system is lesssensitive to media flaws in an individual layer.

Q. Further Comments

While evaluating the materials in the following reported experiments,some further observations were made with respect to certain preferredmaterials according to the present invention. First, the coarsesubstrate provides integrity to the very fine fibers and structure, thusreducing the likelihood that the fine fibers are damaged duringmanufacturing, handling or use. In the absence of the coarse support,fine fiber structure is very easily damaged upon contact with othermaterials. The preferred arrangements of the present invention, however,are often so durable, that it is foreseen some constructions can beprepared which can be washed with liquid or cleaned by an air flush,after use, for some regeneration.

It is also apparent that it may be desirable to create arrangementsaccording the present invention through multiple airlaying of layers,onto a substrate. For example, a porous scrim could be laid down,followed by a fine fiber application, a further coarse layerapplication, a further fine fiber application, etc. Thus, an overallcomposite could be created by a plurality of air laid fiber steps, insequence. This may be desirable in certain processing applications. Itis anticipated that it would not necessarily result in a substantiallydifferent overall operation for the construction, than if it was pairedfrom individual layers.

EXPERIMENTAL

In order to evaluate media according to the present invention, a varietyof experiments were conducted. From the experiments, certain advantagesfrom the use of media according to the present invention will beapparent.

Media Used in the Experiments

For the experiments, a variety of media were utilized. For comparativepurposes in one of the experiments, a 35% LEFS cellulose wet laid mediawas used. The term "35% LEFS" in this context means that the efficiencyof the media for trapping 0.78 micron particles under thecharacterization technique for efficiency described, is 35%.

In some of the experiments, a media wherein the microfibers compriseglass fibers was used. In those instances the media comprised a layer ofglass microfibers on a porous polyester scrim (Reemay 2011). In generalthe glass microfibers were of various diameters between about 0.1 and3.0 microns in diameter. The coarse scrim generally comprised thepolyester scrim described above, commercially available under thedesignation Reemay 2011. The general technique for preparation of thevarious glass fiber samples was described above with respect to U.S.Pat. No. 5,336,286. The glass fiber media or composite is generallycharacterized with respect to %LEFS, with the percent indicatingefficiency for trapping 0.78 micron particles according to thetechniques described herein.

Some of the samples described herein are referred to as "Ultra-Web®type" media or DCI polymeric fiber material. These media generallycomprise the coarse polyester scrim (Reemay 2011) having applied theretomicrofibers of the type used in the Donaldson Company Ultra-Web® surfaceloading media applications. The microfibers are generally of a sizeabout 0.1-0.5 microns, and generally comprise a polymer. The media orcomposite is typically characterized with respect to %LEFS, the termhaving the same meaning as in other applications described above.

Unless stated otherwise, in all experiments the coarse substrate wasReemay 2011, and the composites were laminated using 3M Super 77 sprayadhesive, available from 3M Co., St. Paul, Min.

Experiment 1 Tobacco Smoke Loading

This experiment evaluates use of a high efficiency media using arelatively open pore and fiber structure according to the presentinvention, to improve loading (life) relative to a filter media made ofthe same fine fibers and of approximately the same initial efficiencybut of a smaller interfiber spacing. Tobacco smoke was used for severalreasons. First, it tends to plug conventional high efficiency filters,with small pores, quickly. The tar in the smoke is an amorphous solidthat flows and is subjected to large capillary forces from the smallfibers. The capillary forces cause the tobacco smoke residue to coat thefibers and wick into the pores. Second, it is a common contaminantencountered in vehicle cabin air, indoor air, etc.

Materials Tested

(a) A single layer of Ultra-Web® type fine fiber material comprisinghigh efficiency fine polymeric submicron fibers on a substrate ofHovolin 7311. Hovolin 7311 is a former Hollingsworth & Vose substratecomprising polyester fibers.

(b) A single layer of "medium" efficiency (68.6% LEFS) fine Ultra-Web®fibers that have larger pores (inter fiber spaces) than the single layerof high efficiency fine polymeric fibers in item (a). The substratematerial was Hovolin 7311.

(c) A 14-layer composite of fine polymeric fibers on Reemay 2011.Composite (total initial) LEFS efficiency of 99.6%, single layer LEFSefficiency of approximately 28%. The inter fiber spacing of the finefibers of this material was larger than either of the media described initem (a) or (b). This material was prepared by using an Ultra-Web®technique to apply fine fibers to the Reemay 2011.

Results

1. The loading to 3 inches H₂ O restriction, at 6.8 fpm (2.1meters/min), is measured in number of cigarettes consumed.

2. The area that was loaded was 81 sq.in. in each case.

    ______________________________________                                                Number of     Composite Composite                                             Cigarettes to Initial LEFS                                                                            Final LEFS                                    Media   3.0 in H2O    Efficiency                                                                              Efficiency                                    ______________________________________                                        (a)     3             99.3      not measured                                  (b)     16            68.6      34.1                                          (c)     66            99.6      97.5                                          ______________________________________                                    

3. The final LEFS efficiency being lower than the initial LEFSefficiency (for media (b) and (c)) is believed to be related to thenature of the contaminant. (It is noted that the effect was morepronounced for the single layer system than the multi-layer system.)Fluids which coat fibers effectively increase the wetted fibers'diameter. Also, as small pores are closed and pressure drop increases,flow and aerosols may be diverted to the larger pores which remain openlonger. Relatively small particles (0.78μ) passing through large pores,or past larger wet fibers, would have a lower propensity to collect thanwhen the media is not loaded.

Conclusion

Comparing the differences between media (a) and (b), the tradeoffsbetween life and efficiency are clearly demonstrated and are typical ofthe kinds of choices available to one selecting media for anapplication. In this instance, when going from media (a) to media (b), a5 or 6 fold increase in life was obtained at a cost of passing 45 timesas much contaminant (based on initial efficiency.).

Comparing the differences between media (b) and (c), going from media(b) to media (c), a 4-fold increase in life was obtained and particlepenetration in the composite was reduced by a factor of 78, as reflectedby the initial efficiency increase from 68.6% to 99.6%.

Comparing the differences between media (a) and (c), a 22× increase inlife was obtained and particle penetration for the composite wasessentially unchanged.

For a selected level of efficiency, life can be improved significantlyby using large fibers to space layers of fine fibers relatively farapart. With the present invention, it is possible in some systems toimprove both life and efficiency, or at least to improve one withoutundesirably compromising the other, whereas life is typically traded forefficiency using conventional media.

Experiment 2 DOP Efficiency and Loading

These tests were conducted to evaluate a high efficiency media using arelatively open pore fiber structure to improve loading (life) relativeto a filter media made of the same fine fibers and of approximately thesame initial efficiency but of a smaller interfiber spacing.Alternatively stated, the study was to evaluate whether filter lifeimprovements can be made by reducing the single layer (component)efficiency while maintaining equal composite efficiency. DOP is an oil,not an amorphous solid like the tar contained in cigarette smoke, andacts much like the tar in Experiment 1 with regard to closing pores,wicking, and coating fine fibers. However, the test apparatus used forthis experiment measured real time efficiency and pressure drop.

Set Ub

Ref: MIL STD 282, ASTM D 2986

Materials Tested

(a) A single layer of high efficiency fine polymeric submicron fibers ona substrate of H&V 7311, with a combined LEFS efficiency of 99%.

(b) An 8-layer composite of: 7 layers of fine polymeric fibers on Reemay2011; and, a cover layer of Reemay 2011. The fine fibers were of thetype generated by Donaldson's Ultra-Web® processing. The composite(total) initial LEFS efficiency was 97.5%, with a single layer (Reemay2011 with fine fiber) LEFS efficiency of 41%. The inter fiber spacing ofthe fine fibers of this material was observed to be larger than that ofth media described in item (a).

(c) A 14-layer composite of fine polymeric fibers (Ultra-Web®-typefibers) on Reemay 2011. The composite had a total initial LEFSefficiency of 99%, and a single layer (Reemay 2011 with fine fibers)LEFS efficiency of 28%. The inter fiber spacing of the fine fibers ofthis material was designed to be larger than either of the mediadescribed in item (a) or (b).

Results

The media of (b) and (c) showed significant loading advantages over themedia made from relatively closely spaced fine polymer fibers (i.e.media (a)).

    ______________________________________                                                         Final                  Single                                        Composite                                                                              dP After               Layer                                         LEFS     100 mg   Initial                                                                              Dp     LEFS                                  Media   Efficiency                                                                             DOP      Dp     Rise   Efficiency                            Description                                                                           (%)      (in H.sub.2 O)                                                                         (in H.sub.2 O)                                                                       (in H.sub.2 O)                                                                       (%)                                   ______________________________________                                        1-layer fine                                                                          99%      1.95     0.40   1.55   99%                                   polymeric                                                                     fiber                                                                         8-layer fine                                                                          97.5%    0.70     0.62   0.08   40%                                   polymeric                                                                     fiber                                                                         14-layer fine                                                                         99.0%    0.70     0.58   0.12   28%                                   polymeric                                                                     fiber                                                                         ______________________________________                                    

The loss in efficiency with time is consistently observed with all ofthe samples of this test, and is analogous to the reduction in LEFSefficiencies reported in the tobacco loading experiment (Experiment 1).The reason for the reduction in efficiency is believed to be caused bythe same phenomenon experienced in the tobacco smoke tests, which wereexplained in the conclusions to Experiment 1.

    ______________________________________                                                                  Final DOP                                                                     Efficiency    Single                                        Composite                                                                              Initial  After  DOP    Layer                                         LEFS     DOP      100 mg Efficiency                                                                           LEFS                                  Media   Efficiency                                                                             Efficiency                                                                             DOP    Loss   Efficiency                            Description                                                                           (%)      (% absol)                                                                              (% absol)                                                                            (% absol)                                                                            (%)                                   ______________________________________                                        1-layer fine                                                                          99%      72       62     16     99%                                   polymeric                                                                     fiber                                                                         8-layer fine                                                                          97.5%    59       56     3      40%                                   polymeric                                                                     fiber                                                                         14-layer fine                                                                         99.0%    83       70     13     28%                                   polymeric                                                                     fiber                                                                         ______________________________________                                    

NOTE: The multi-layer spaced, fine fiber structure experienced apressure drop increase of 1/20th (media (b)) and 1/13th (media (c)) ofthe closely spaced fine fibers of media (a).

Conclusion

DOP loading results are consistent with the tobacco smoke loadingresults. The sum of low efficiency layers resulting in alternatingfine-coarse fiber composite structure provides substantial loading(life) benefits over a filtration media with a single fine fiber layerefficiency approximately equivalent to the combined layers of thecomposite.

Experiment 3 NaCl Loading

This series of tests was performed to evaluate filter life benefits thatcan be obtained by going from a single high efficiency layer of finefiber media supported by a substrate to multiple low efficiency layersof fine fiber on a substrate, with approximately equal composite LEFSefficiencies. These tests are distinguished from the tobacco and DOPloading in that the salt particles fed to the media are discrete solidparticles, not liquid or semi-amorphous solids, therefore caking occursand efficiency increases with loading. It was observed that after cakingoccurs, the slopes of the loading curves were very similar for all ofthe media tested. In particular, after caking occurs, the media is nolonger being tested/challenged.

One measure of filter life is time to a predetermined pressure drop;another is mass of contaminant fed to a predetermined pressure drop. Ifthe predetermined terminal pressure drop is significantly above therestriction where cake formation begins, then cake loading comparisonsare being made rather comparisons between media performance. Lifecomparisons here are made at a restrictions where cake formation isnormally completed. In the following tables operation to 2 inches H₂ Oand 5 inches H₂ O are given. Samples were tested at 10 fpm.

Set Up

Media were loaded on a custom made salt loading bench (schematic shownin FIG. 25), using commercially available components, specifically a TSIconstant high output atomizer model 3076 for particle generation, a TSImodel 3054 aerosol neutralizer, and a TSI Electrical Aerosol Analyzer(EAA) model 3030 used for particle counting and sizing to measureparticle efficiency as test media is loaded. Submicron salt was used asthe contaminant, because it is more easy to discern loading differencesbetween various media when this contaminant is used, than whentraditional SAE silica dust is used.

Materials Tested

Media with composite LEFS efficiencies of 40 to 45%, 60 to 65%, and 75to 80%. All composites were made from polymeric fine fibers(Ultra-Web®-type fibers) on a Reemay 2011 substrate. For a givencomposite sample, all layers within that sample were made from media ofequal LEFS efficiency media (i.e. there were no efficiency gradients inthe composites tested in this series of experiments). For instance, if acomposite had an LEFS efficiency of 50% and was made of 6 layers, eachlayer (Reemay 2011 substrate with fine fibers thereon) would have anLEFS efficiency of 10.9%.

Results

Composite media with lower layer LEFS efficiency have better loading(life) than composite media comprised of a fewer layers with a higherlayer LEFS efficiency. This apparent ability to choose both efficiencyand life, independently, differs from many applications of traditionalmedia. With many practices using conventional media, life is gained bysacrificing efficiency.

    ______________________________________                                        40-45% Composite LEFS Efficiency                                              Number   Single Layer                                                                              Time       Life Relative to                              of       LEFS        to 5 in H.sub.2 O                                                                        Reference Media                               Layers   Efficiency  (min)      (min/min)                                     ______________________________________                                        1.sup.  (Ref Media)                                                                    41%         230        100%                                          2        21          280        122%                                          3        16          350        152%                                          4        12          630        273%                                          5        10          680        295%                                          ______________________________________                                        40-45% Composite LEFS Efficiency                                              Number   Single Layer                                                                              Time       Life Relative to                              of       LEFS        to 2.0 in H.sub.2 O                                                                      Reference Media                               Layers   Efficiencv  (min)      (min/min)                                     ______________________________________                                        1.sup.  (Ref Media)                                                                    41%         110        100%                                          2        21          170        154%                                          3        16          250        227%                                          4        12          475        432%                                          5        10          525        477%                                          ______________________________________                                        60-65% Composite LEFS Efficiency                                              Number   Single Layer                                                                              Time       Life Relative to                              of       LEFS        to 5 in H.sub.2 O                                                                        Reference Media                               Layers   Efficiency  (min)      (min/min)                                     ______________________________________                                        2.sup.1 (Ref Media)                                                                    41% (each layer)                                                                          165        100%                                          3        28          230        139%                                          4        21          290        176%                                          5        18          305        185%                                          7        13          510        309%                                          9        10          660        400%                                          ______________________________________                                         .sup.1 For this sample (and other samples where the reference layer is        identified as having two layers) a twolayer composite was used as the         reference media, because a single layer of 60-65% LEFS was not                conveniently available. Since the composite comprised two layers, each of     which was 41% efficient, the overall efficiency was 1 -  (1 - .41) .times     (1 - .41) or 65%.                                                        

    60-65% Composite LEFS Efficiency                                              Number   Single Layer                                                                              Time       Life Relative to                              of       LEFS        to 2.0 in H.sub.2 O                                                                      Reference Media                               Layers   Efficiency  (min)      (min/min)                                     ______________________________________                                        2.sup.  (Ref media)                                                                    41% (each layer)                                                                           85        100%                                          3        28          125        147%                                          4        21          175        205%                                          5        18          210        250%                                          7        13          375        440%                                          9        10          540        635%                                          ______________________________________                                        70-80% Composite LEFS Efficiency                                              Number   Single Layer                                                                              Time       Life Relative to                              of       LEFS        to 5 in H.sub.2 O                                                                        Reference Media                               Layers   Efficiency  (min)      (min/min)                                     ______________________________________                                        3.sup.  (Ref Media)                                                                    40%         170        100%                                          4        28%         230        135%                                          6        20%         260        150%                                          7        18%         340        200%                                          8        16%         410        240%                                          9        13%         540        320%                                          ______________________________________                                        70-80% Composite LEFS Efficiency                                              Number   Single Layer                                                                              Time       Life Relative to                              of       LEFS        to 2.0 in H.sub.2 O                                                                      Reference Media                               Layers   Efficiency  (min)      (min/min)                                     ______________________________________                                        3.sup.2 (Ref Media)                                                                    40% (each layer)                                                                           80        100%                                          4        30%         125        156%                                          6        20%         170        213%                                          7        18%         230        287%                                          8        16%         280        350%                                          9        13%         400        500%                                          ______________________________________                                         .sup.2 Here, a threelayer system was used as the reference. It had an         overall composite efficiency of 78%.  1 - (1 - .40).sup.3                

Conclusion

From the results of this experiment, it is clear that is it possible tochoose both a medium's efficiency and loading (life) independently,whereas for typical conventional media and a selected initial LEFSefficiency, the corresponding range of salt loading life values might belimited to a range of less than 2:1. This experiment has demonstratedthe ability to increase sub micron salt loading life by a factor of 5 or6, through providing increased spacing between layers of fine fibers.

Experiment 4 Salt Loading at 150 fpm

Set Up

Sample Area: 25 square inches (flat square sheet) using a customschematic 14 custom Collison atomizers, TSI 3054 neutralizer

Materials Tested

1. Conventional wet-laid cellulose used in engine air filtration with aninitial LEFS efficiency of 35-38%. Typically operates between 8 and 10fpm face velocity.

2. A 3-layer composite made from wet-laid hand sheets of glassmicrofibers that range in size from submicron to about 3 micron onReemay 2011.

overall composite LEFS efficiency 32%

single layer efficiency 12% (each)

The glass fibers used were Schuller #106.

Results

    ______________________________________                                                          Initial  Initial      Time at                                        Thick-   LEFS     Dp at  dP    150 fpm                                        ness     Efficiency                                                                             150 fpm                                                                              Rise (in                                                                            to dP Rise                            Media    (inch)   (%)      (in H.sub.2 O)                                                                       H.sub.2 O)                                                                          (in H.sub.2 O)                        ______________________________________                                        Cellulose                                                                              .013-.015                                                                              35-38%   6.8    5.8   13                                    surface                                                                       loading media                                                                 3-Layer  .026-.028                                                                              32%      0.5    3.5   75                                    composite                                                                     (glass fibers;                                                                no gradient)                                                                  ______________________________________                                    

Conclusion

A 3-layer pleatable composite media, including scrim having a web madefrom fine glass fibers (submicron -3 microns in diameter), demonstrateda significantly greater permeability (13×) and submicron salt loadinglife (>5×) than a pleatable cellulose surface loading media ofapproximately equal initial LEFS efficiency. The test velocity of 150fpm was arbitrary and intended to illustrate a capability of the media.This is not meant to suggest that engine air cellulose media normallyoperates at 150 fpm face velocity.

Experiment 5 Non-Gradient vs Gradient Embodiment

This experiment was intended to compare the loading results of agradient embodiment of the invention with an initial LEFS efficiency ofabout 65% to that of a non-gradient media of equal number of layers andequal LEFS efficiency.

Set Up

Same as salt loading for Experiment 3 above.

Materials Tested

1. The non-gradient media was made from submicron polymeric fiber(Ultra-Web®-type fiber) deposited onto Reemay 2011 and laminated by handusing 3M Super 77, each layer having an approximately equal LEFSefficiency to the other two layers in the composite. The single layerLEFS efficiency was about 24%.

2. The gradient media was made from submicron polymeric fibers(Ultra-Web®-type fibers) deposited onto Reemay 2011 with succeedinglayers having greater LEFS efficiency than the preceding layers. Thegradient chosen was arbitrary, and it is not known if additional lifebenefits would have been gained with a different selection of layers forthe 3 layer gradient composite, at the same overall LEFS efficiency. Inthis instance, from upstream to downstream, the LEFS efficiencies of thelayers were about 10%, 20%, and 40%. These too were hand laminated using3M Super 77.

Results

The results are as shown in FIG. 22.

Observations and Conclusion

The gradient version of the invention better utilized the availablemedia volume than a non-gradient equivalent (thickness, perm, and LEFSefficiency). This is demonstrated by a 66% increase in submicronparticle (NaCl) loading of the gradient sample relative to thenon-gradient. This again can be explained in terms of the interfiberspacing of the fine fibers. A non-gradient media structure of the samevolume and efficiency as a gradient media is more likely to not utilizethe loading potential of the fibers towards the downstream side of themedia due to cake formation on the upstream side of the non-gradientmedia. A cake forms sooner on the non-gradient media than the gradientmaterial. This is due to the average distance between fine fibers beingsmaller for the non-gradient media than that of the lower efficiencyupstream layers of the gradient media. Using LEFS efficiency as an indexfor fiber spacing, the first layer of the non-gradient arrangement has a24% LEFS efficiency, whereas the gradient structure's first layer is 10%efficient. Therefore, a gradient media structure will tend to moreeffectively utilize all of the available media volume than anon-gradient equivalent.

Experiment 6

Comparison Between Glass and Polymeric Submicron Fibers

This test compares the gradient media tested in Experiment 5 which useda submicron polymeric (Ultra-Web ®-type) fine fiber, with a glass fibersystem. The polymeric fibers were about 0.4μ with a relatively smallfiber size variance. The glass fibers were about 0.2 to 3.0μ, Schullerfiber 106. Single layer media of 40% LEFS efficiency were also tested inthe glass and polymeric versions.

Set Up

See 10 fpm salt loading Experiment 3.

Materials Tested

1. Polymeric fine fiber gradient media from Experiment 5.

2. A submicron glass fiber gradient version of the gradient media testedin Experiment 5. The submicron glass fibers were selected to match themedian size as the polymeric fiber but having a different distributionabout the mean. Wet-laid handsheets were prepared using a standard 8×8inch handsheet former. The glass fibers were placed onto the Reemay 2011which was supported by a fine plastic mesh which normally collectsfibers drained from the slurry.

3. Single layer of fine fiber polymeric media with a 40% LEFSefficiency.

4. Single layer of fine fiber glass (Schuller #106) media with a 40%LEFS efficiency.

Results

The difference in loading for the gradients was about 5% at 5.0 in H₂ O,and the loading of single layer 40% LEFS samples differed by about 10%.The rate at which the media seasoned were very similar for the singlelayer media. For the gradient media, the glass fiber sample increased inefficiency faster than the polymeric fiber version. The reason for thiswas partially understood at a later point in time when it was discoveredthat glass fibers of up to about 3μ were included in the glass fiberstock used to make handsheet samples. This was discovered when SEMS weretaken for pore size analysis. The results of this experiment are plottedin FIGS. 23 and 23A.

In FIG. 23, the plot compares the performance of the single layerpolymeric fiber version (40% LEFS) with the single layer glass fiberversion (40% LEFS).

In FIG. 23A, the plot compares the performance of the 3-layer gradientpolymeric version (60% LEFS) to the 3-layer gradient glass fiber version(60% LEFS). Note also that for each type (polymer or glass) the media inthe form of a gradient system had about 70% more life and about a 33%reduction in penetration. This indicates that efficiency does not haveto be sacrificed to gain life, when preferred techniques of the presentinvention are used.

Conclusion

Gradient forms of the media tend to load better than non-gradientsystems.

Before cake formation, glass and polymeric fibers perform verysimilarly, though the glass fibers included a broader fiber sizedistribution than the polymeric fibers. Given the disparity in fibersize and distribution, this was unexpected. The difference in slope ofthe loading curve after cake formation is not presently understood.

Experiment 7 Observations of Various Samples

FIGS. 12-21 are scanning electron micrographs (SEMs) of various media.The principles according to the present invention can be understood byreviewing the various media depicted.

Attention is first directed to FIG. 12. FIG. 12 is a scanning electronmicrograph, 100× magnification, which shows a conventional air laidpolymeric fiber media, in particular Kem Wove 8643. Consistency of fibersize is observable. This is a 1.5 denier material. Its LEFS efficiencyis 3%. Its thickness is about 0.30 inches. It has a basis weight ofabout 73 lb/3000 ft², a volume solidity of 1.1% and a permeability of400 fpm.

FIG. 13 is a convention air laid glass fiber media, at 100×magnification. The particular media is AF18, available from Schuller.Again, consistency of fiber size is viewable. It has an LEFS efficiencyof 12%; a thickness of 0.18 inches; a basis weight of 60 lb/3000 ft² ; avolume solidity of 0.9%; and a permeability of 230 fpm. The material hasa 45% ASHRAE rating and an approximate fiber size of 4.5μ.

FIGS. 14 and 15 depict a conventional two-phase media, at 500-foldmagnification. Both phases are glass fibers. The media of the twophotographs is Hollingsworth & Vose HF343. FIG. 14 is of the upstreamside, where the more coarse fibers are located. FIG. 15 is of thedownstream side, and a mixture of the finer fibers with the coarse isviewable. HF343 is a wet-laid glass fiber media. The upstream side ofthe media (phase one) has relatively open, large, coarse,self-supporting fibers intended to capture and store coarse contaminant.The downstream side (second phase) of the media is made of a combinationof fine and coarse fibers. The fine fibers provide higher efficiency butlower capacity than the large fibers in phase one. The media has anASHRAE rating of approximately 60-65%. HF343 has an LEFS efficiency of23%; thickness of 0.02 inches; a basis weight of 50 lb/3000 ft² ; avolume solidity of 7.1%; and a permeability of about 135 fpm.

In general, the volume solidity of a fine fiber layer, of the presentinvention, is difficult to measure directly, or indirectly, and becomesmore difficult for LEFS efficiencies less than about 15-20%. The primarydifficulty lies in estimating the normal local thickness of the finefiber layer. For a typical combination of fine and coarse fibers used toconstruct arrangements according to the present invention, the finefibers create a open porous "surface". The topography of the surfaceresembles that of a spider web draped over a support structure. Thesurface of the microfiber matrix derives its shape from the fiberstructure and voids beneath it (the support structure), consequently thematrix has many peaks, valleys, ridges and troughs. The thicknessdimension used for estimating the solidity is not the dimension from apeak to a valley, but the thickness of the web/layer at a peak, at avalley, or at a local planer region. This geometry has features that arenot evident in SEM photos, but are readily apparent when inspectedthrough a stereoscope, at 10× to 40× magnification. The solidityestimates reported for the materials of the invention are derived fromestimates of the local thickness normal to fine fiber layer.

FIG. 16 is a composite media according to the present invention. Themedia comprises Schuller glass fiber 106 deposited on Reemay 2011. Thefine fiber diameter range extended from submicron up to about 3 microns.The amount of fiber 106 deposited is sufficient for the resulting layerto have a percent efficiency LEFS of 40%. In the picture, the very finefibers comprising the layer of fine fibers are readily viewable.Underneath, in some locations, the more coarse fibers can be viewed.

The material of FIG. 16 was made of a wet-laid hand sheet of the finefiber material, deposited onto a Reemay 2011 substrate as describedabove in the specification. In this media, the figure that wouldrepresent the ratio of the fiber diameter of the coarse substrate fibersto the fiber diameter of the fine fibers is much greater, than in themedia depicted in FIGS. 14 and 15. The fine fiber layer permeability isestimated by removing the substrate contribution from the permeabilityof the composite. For low efficiency-high permeability samples, it wasnecessary to stack multiple layers to obtain measurable values tocompute average permeability.

For this material, when measured at 1000×, the average area solidity wasabout 52%. The permeability was about 10 fpm, the volume solidity about10%, the basis weight 1.5 lb/3000 ft², and the thickness 10 microns.

FIG. 17 is another composite media according to the present invention.The media in FIG. 17 is shown at 100× magnification. The media comprisesa DCI (Donaldson Company Inc.) polymeric fine fiber positioned on acoarse substrate comprising Reemay 2011. The DCI polymeric fine fiberwas made generally according to the same process used to form finepolymeric fibers for Donaldson's Ultra-Web® products, a trade secretprocess. The fine fiber diameter was submicron.

FIG. 18 is another composite media according to the present invention.In FIG. 18, the media is shown at 100-fold magnification. The mediacomprises Schuller glass fiber 106 deposited on Reemay 2011. The amountof glass fiber present was sufficient to provide an efficiency (% LEFS)of 12%. The basis weight of the fine fiber layer was about 0.5 lb/3000ft², and the permeability was about 600 fpm. In the micrograph, both thecoarse fibers and the fine fibers are readily discernable. This materialhad an average area solidity of about 33% when evaluated at 1000×magnification.

FIG. 19 is another composite media according to the present invention.It comprises DCI polymeric fine fiber deposited on Reemay 2011, depictedat 100-fold magnification. The media depicted had a percent efficiencyof 12% LEFS. Again, the web of fine fibers is readily discernablepositioned on top of the underlying coarse fiber support. When evaluatedat 500× magnification, this material was observed to have an averagearea solidity of 22%.

FIG. 20 is a micrograph of the material shown in FIG. 19, depicted at500× magnification. The very fine fiber web, on top of the underlyingcoarse fiber support, is readily discernable.

FIG. 21 is a 1000× magnification of the material shown in FIG. 19, afterNaCl loading. The salt particles, trapped on the very fine fibers, arereadily discernable on the picture.

In FIG. 24, NaCl loaded 18% LEFS media is shown, at 1000× magnification.The NaCl particles are viewable primarily trapped on the fine fibers.The media material of FIG. 24 is DCI fine fiber polymer on a Reemay 2011coarse substrate.

In the arrangements according to the present invention, depicted inFIGS. 16-20, the characteristic of very fine fibers being positioned ontop of a coarse substrate is generally discernable. This is the caseregardless of the percent efficiency, or the particular materialutilized for formation of the fine fibers. In FIG. 21, operation toachieve load on the fine fiber was readily observed.

FURTHER OPTIONS

It is foreseen that in some instances arrangements according to thepresent invention may be utilized in an environment involving thefiltering of fluid streams which contain components that are chemicallyincompatible with certain types of fiber materials. For example, someair streams may carry chemicals which are damaging to polymericmaterials, but not damaging to glass. If such is the case, it will bepreferred to construct at least the fine fibers of the filteringmaterial from a composition which is resistant to damage under theintended use environment.

Further, it is foreseen that it may be desirable in some circumstancesto utilize the present invention in a "scaled up" version. This would bean application in which the "fine fibers" of the composite arerelatively large and the coarse fibers are even larger. That is, theratio of size between the fine fibers and the coarse fibers would bemaintained within the ranges generally stated herein, however the sizeof each would be substantially larger than the preferred rangesdisclosed herein. For example, each might be 5× to 10× larger thandefined herein. Such constructions may be usable, for example, in uniqueenvironments involving the filtering of rather large particles. It isnot anticipated that such constructions will be preferred or desirablefor most typically encountered industrial and/or engine environments.

What is claimed is:
 1. An air filter construction comprising:(a) apleated filter media arrangement comprising a multi-layer compositehaving a thickness of no greater than about 0.15 cm, a pleat depth of atleast 0.6 cm, and including more than one pleat per 2.54 cm; saidpleated media arrangement including first and second layers of finefiber media therein:(i) said first layer of fine fiber media comprisinga most upstream layer of fine fiber media positioned within said pleatedmedia; said first layer of fine fiber media comprising media having anaverage fiber diameter of no greater than about 3 microns and said firstlayer comprising fibers with diameters no greater than about 5 microns;(ii) said first layer of fine fiber media having a thickness of nogreater than 15 microns; (iii) said first layer of fine fiber mediahaving a permeability, on its own, of at least about 90 meters/min.;and, an efficiency of no greater than about 30%, for 0.78 micronsmono-dispersed polystyrene latex spheres; (iv) said second layer of finefiber media being positioned downstream from said first layer of finefiber media, said second layer of fine fiber media comprising mediahaving an average fiber diameter of no greater than about 3 microns andsaid second layer comprising fibers with diameters no greater than about5 microns; (v) said second layer of fine fiber media having a thicknessof no greater than 15 microns; (vi) said second layer of fine fibermedia having an efficiency of no greater than about 60%, for 0.78 micronmono-dispersed, polystyrene latex spheres; (vii) said second layer offine fiber media having a greater efficiency for 0.78 micronmono-dispersed polystyrene latex spheres, than said first layer of finefiber media; (viii) said second layer of fine fiber media being spacedfrom said first layer of fine fibers by a distance of no greater than254 microns; (b) said pleated media arrangement including a firstspacing structure comprising a region of coarse fiber materialpositioned to separate said first layer of fine fiber media from saidsecond layer of fine fiber material; said first spacing structurecomprising a material having an efficiency on its own of no greater than10%, for 0.78 micron mono-dispersed polystyrene latex spheres;(i) saidfirst spacing structure having a permeability of at least 200meters/min.; (ii) said first spacing structure comprising a nonwovensubstantially continuous fiber matrix having an average fiber diameterof at least 12 microns; (c) said pleated media arrangement including anupstream layer of coarse fiber material positioned adjacent an upstreamside of said first layer of fine fiber media; said upstream layer ofcoarse fiber material comprising a nonwoven substantially continuousfiber matrix having an average fiber diameter of at least 12 microns;(i)said upstream layer of coarse fiber material having an efficiency, onits own, of no greater than 10%, for 0.78 micron mono-dispersedpolystyrene latex spheres; and, (d) said pleated media arrangementincluding a downstream layer of coarse fiber material positionedadjacent a downstream side of said second layer of fine fiber media;said downstream layer of coarse fiber material comprising a nonwovensubstantially continuous fiber matrix having an average fiber diameterof at least 12 microns;(i) said downstream layer of coarse fibermaterial having an efficiency, on its own, of no greater than 10%, for0.78 micron mono-dispersed polystyrene latex spheres.
 2. An air filterconstruction according to claim 1 including:(a) a second filterpositioned upstream of said pleated filter media arrangement.
 3. An airfilter construction according to claim 2 wherein:(a) said second filtercomprises depth media.
 4. An air filter construction according to claim1 wherein:(a) said pleated filter media comprises a cylindrical pleatedfilter.
 5. An air filter construction according to claim 1 wherein:(a)said pleated filter media comprises a panel filter.
 6. An air filterconstruction according to claim 1 wherein:(a) said pleated filter mediaincludes a third layer of fine fiber media positioned downstream fromsaid second layer of fine fiber media;(i) said third layer of fine fibermedia being spaced from said second layer of fine fiber media by adistance of no greater than 254 microns; (ii) said third layer of finefiber media comprising media having an average fiber diameter of nogreater than about 3 microns and comprising fibers with diameters of nogreater than about 5 microns; (iii) said third layer of fine fiber mediahaving a thickness of no greater than 15 microns; (iv) said third layerof fine fiber media having an efficiency of no greater than about 70%,for 0.78 micron mono-dispersed polystyrene latex spheres; (v) said thirdlayer of fine fiber media having a greater efficiency for 0.78 micronmono-dispersed polystyrene latex spheres, than either one of said firstand second layers of fine fiber media.
 7. An air filter constructionaccording to claim 6 wherein:(a) said pleated filter media includes afurther layer of coarse fiber material;(i) said further layer of coarsefiber material being positioned adjacent a downstream side of said thirdlayer of fine fiber media; (ii) said further layer of coarse fibermaterial comprising a nonwoven substantially continuous fiber matrixhaving an average fiber diameter of at least 12 microns; and, (iii) saidfurther layer of coarse fiber material having an efficiency, on its own,of no greater than 10%, for 0.78 micron mono-dispersed polystyrene latexspheres.
 8. An air filter construction according to claim 1 wherein:(a)said first layer of the fine fiber media comprises glass fibers.
 9. Anair filter construction according to claim 1 wherein:(a) said firstlayer of fine fiber media has a thickness of no greater than about 4microns.
 10. An air filter construction according to claim 9 wherein:(a)said second layer of fine fiber media has a thickness of no greater thanabout 4 microns.
 11. An air filter construction according to claim 1wherein:(a) the first spacing structure comprises polyester media havinga basis weight of no greater than 45.0 g/m².
 12. An air filterconstructing according to claim 1 wherein:(a) said first layer of finefiber media has a basis weight of no greater than about 1.55 g/m². 13.An air filter construction according to claim 12 wherein:(a) said secondlayer of fine fiber media has a basis weight of no greater than about1.55 g/m².
 14. An air filter construction according to claim 1wherein:(a) said first spacing layer comprises polyester fibers havingdiameters within the range of 25 to 35 microns.
 15. An air filterconstruction comprising:a pleated filter media arrangement comprising amultilayer composite having a thickness of no greater than about 0.15cm, a pleat depth of at least 0.6 cm, and including more than one pleatper 2.54 cm; said pleated media arrangement including first, second andthird layers of fine fiber media therein;(i) said first layer of finefiber media comprising a most upstream layer of fine fiber mediapositioned within said pleated media; said first layer of fine fibermedia comprising media having an average fiber diameter of no greaterthan about 3 microns and said first layer comprising fibers withdiameters no greater than about 5 microns; (ii) said first layer of finefiber media having a permeability, on its own, of at least about 90meters/min.; and, an efficiency of about 10%, for 0.78 micronmono-dispersed polystyrene latex spheres; (iii) said second layer offine fiber media being positioned downstream from said first layer offine fiber media, said second layer of fine fiber media comprising mediahaving an average fiber diameter of no greater than about 3 microns andsaid second layer comprising fibers with diameters no greater than about5 microns; (iv) said second layer of fine fiber media having anefficiency of about 20%, for 0.78 micron mono-dispersed, polystyrenelatex spheres; (v) said third layer of fine fiber media being positioneddownstream from said second layer of fine fiber media; said third layerof fine fiber media comprising media having an average fiber diameter ofno greater than about 3 microns and said third layer comprising fiberswith diameters no greater than about 5 microns; (vi) said third layer offine fiber media having an efficiency of about 40%, for 0.78 micronmono-dispersed, polystyrene latex spheres; (b) said pleated mediaarrangement including a first spacing structure comprising a region ofcoarse fiber material positioned to separate said first layer of finefiber material from said second layer of fine fiber material; said firstspacing structure comprising a material having an efficiency, on itsown, of no greater than 10%, for 0.78 micron mono-dispersed polystyrenelatex spheres;(i) said first spacing structure having a permeability ofat least 200 meters/min.; (ii) said first spacing structure comprising anonwoven, substantially continuous, fiber matrix having an average fiberdiameter of at least 12 microns; and, (c) said pleated media arrangementincluding a second spacing structure comprising a region of coarse fibermaterial positioned to separate said second layer of fine fiber materialfrom said third layer of fine fiber material; said second spacingstructure comprising a material having an efficiency, on its own, of nogreater than 10%, for 0.78 micron mono-dispersed polystyrene latexspheres;(i) said second spacing structure having a permeability of atleast 200 meters/min.; (ii) said second spacing structure comprising anonwoven, substantially continuous, fiber matrix having an average fiberdiameter of at least 12 microns.
 16. An air filter constructionaccording to claim 15 wherein:(a) said pleated media arrangementincludes an upstream layer of coarse fiber material positioned adjacentan upstream side of said first layer of fine fiber media; said upstreamlayer of coarse fiber material comprising a nonwoven substantiallycontinuous fiber matrix having an average fiber diameter of at least 12microns;(i) said upstream layer of coarse fiber material having anefficiency, on its own, of no greater than 10% for 0.78 micronmono-dispersed polystyrene latex spheres.
 17. An air filter constructionaccording to claim 16 wherein:(a) said pleated media arrangementincludes a downstream layer of coarse fiber material positioned adjacenta downstream side of said third layer of fine fiber media; saiddownstream layer of coarse fiber material comprising a nonwovensubstantially continuous fiber matrix having an average fiber diameterof at least 12 microns;(i) said downstream layer of coarse fibermaterial having an efficiency, on its own, of no greater than 10% for0.78 micron mono-dispersed polystyrene latex spheres.
 18. An air filterconstruction according to claim 17 wherein:(a) said first, second andthird layers of fine fiber media comprise glass fiber media.
 19. An airfilter construction according to claim 17 wherein:(a) said pleatedfilter media comprises a cylindrical pleated filter.
 20. An air filterconstruction according to claim 17 wherein:(a) said pleated filter mediacomprises a panel filter.